American Diploma · ACT · English: Sentence Structure

Run-ons and Comma Splices

How to spot run-ons and comma splices and the five correct ways to join two independent clauses — including however and therefore — with worked examples and a 10-question ACT-style practice.

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What this question type tests

Joining clauses is one of the most frequent skills in the Conventions of Standard English reporting category of the ACT English test (38–43% of the English score, together with punctuation and usage). The English test has 50 questions (40 scored) in 35 minutes over five essays: three of about 340 words with 10 questions each and two of about 185 words with 5 questions each.

These items look like punctuation questions: the underlined portion sits where two ideas meet, and the four choices (the first usually NO CHANGE) swap a comma, a period, a semicolon, a colon, a FANBOYS word, or a word such as however, because, which. Sometimes the stem asks which choice is NOT acceptable — then three choices join the clauses correctly in different ways, and you must find the one broken version. Either way, the question is the same: how many complete sentences are there, and are they joined legally?

Two kinds of clause

A clause is a group of words with a subject and a finite verb.

An independent clause can stand alone as a sentence: ✓ *the ferry left the harbour*.
A dependent clause starts with a word such as because, although, when, while, if, since, which, who, that — it cannot stand alone: *because the storm passed*, *which carries 300 passengers*.

Every rule on this page is about one situation: two independent clauses side by side.

The two errors: run-ons and comma splices

Error	What it looks like	✗ Example
Run-on (fused sentence)	Two independent clauses with no punctuation at all	The museum opened in 2004 it now holds two million objects.
Comma splice	Two independent clauses joined by a comma alone	The museum opened in 2004, it now holds two million objects.

The ACT tests the comma splice far more often than the fused sentence, because a comma "looks" right. Train your eye: when a comma is followed by a new subject and verb, stop and check.

Splice warning words. A second clause that begins with it, this, that, they, he, she, we, there, these, those or a name is almost always a new independent clause. A comma alone is never enough before it.

✗ The bridge was closed for repairs, this caused long delays.
✓ The bridge was closed for repairs, which caused long delays. (which = dependent clause)
✗ Nadia loves chemistry, she spends every weekend in the lab.
✓ Nadia loves chemistry, so she spends every weekend in the lab.

الغلطة الأشهر

فاصلة لوحدها بين جملتين كاملتين = comma splice = غلط. لو بعد الفاصلة لقيت it أو this أو they أو he أو she وبعدها فعل، اعرف إن دي جملة جديدة ومحتاجة نقطة أو ; أو فاصلة + and/but/so.

The five correct ways to join two independent clauses

Way	Pattern	✓ Example
1. Period	Clause. Clause.	The storm passed. The ferry left the harbour.
2. Semicolon	Clause; clause.	The storm passed; the ferry left the harbour.
3. Comma + FANBOYS	Clause, and/but/so… clause.	The storm passed, so the ferry left the harbour.
4. Colon (2nd explains the 1st)	Clause: clause.	The captain had one rule: no ferry left during a storm.
5. Make one clause dependent	Because/When/Although… clause, clause. — or — Clause because… clause.	When the storm passed, the ferry left the harbour. / The ferry left because the storm passed.

1. Period. Always correct grammatically. On the ACT, a period and a semicolon are interchangeable — if one is right, the other is usually right too, so neither can be the single answer when both appear.

2. Semicolon. The teacher's rule: a semicolon joins two independent clauses that are closely related. Both sides must be complete sentences — if you could not put a period there, you cannot put a semicolon there.

✓ The road was flooded; the buses took a longer route.
✗ The road was flooded; because it rained all night. (right side is dependent)
✗ Because it rained all night; the road was flooded. (left side is dependent)

(SAT/ACT extra rule: a semicolon may also separate list items that already contain commas — "Cairo, Egypt; Rabat, Morocco; and Tunis, Tunisia." That is its only other use, and it has nothing to do with joining clauses.)

3. Comma + FANBOYS. The seven coordinating conjunctions are For, And, Nor, But, Or, Yet, So. Put the comma before the FANBOYS word.

✓ The tickets were cheap, yet few people came.
✗ The tickets were cheap yet, few people came. (comma in the wrong place)
✓ She finished the essay and sent it to her teacher. (one subject, two verbs — NO comma, because "sent it" is not a clause)
✗ She finished the essay, and sent it to her teacher.

That last pair is a favourite ACT trick: a comma + FANBOYS is only for two complete clauses. If the second part has no subject of its own, do not add a comma.

4. Colon. Use a colon after a complete sentence when what follows explains it, gives emphasis or adds information. The left side must be complete.

✓ The answer was obvious: the pump had never been switched on.
✗ The reason was: the pump had never been switched on. ("The reason was" is not complete)

5. Make one clause dependent. Add a subordinating word (because, although, when, while, since, if, unless, after, before) or turn the second clause into a which/who clause. If the dependent clause comes first, put a comma after it; if it comes second, usually no comma (though a comma before *which* or *although* is normal).

✓ Although the tickets were cheap, few people came.
✓ The museum opened in 2004, which makes it one of the newest in the city.
✗ The museum opened in 2004, it makes it one of the newest in the city.

Conjunctive adverbs: however, therefore and friends

Words such as however, therefore, moreover, consequently, nevertheless, thus, instead, still, otherwise, meanwhile, as a result, in addition, for example, in fact connect ideas, but they are not FANBOYS. They cannot join two clauses with only a comma.

✗ The road was closed, therefore, we walked. (comma splice)
✓ The road was closed; therefore, we walked.
✓ The road was closed. Therefore, we walked.
✓ The road was closed, so we walked. (FANBOYS instead)
✓ We, therefore, walked home. (only one clause — the adverb is extra information inside a pair of commas)

How to decide: look at what is on each side of the adverb. If there is a complete clause on both sides, you need a semicolon or period before it and a comma after it. If the adverb sits inside one clause, a pair of commas is fine.

✓ The plan, however, failed. (one clause)
✗ The plan was good, however, it failed. (two clauses)
✓ The plan was good; however, it failed.

"Then" is a trap too. It is not a FANBOYS word. ✗ We packed the car, then we drove to Alexandria. ✓ We packed the car, and then we drove to Alexandria. ✓ We packed the car; then we drove to Alexandria.

however و therefore

كلمات زي however و therefore و moreover مش زي and و but. لو قبلها جملة كاملة وبعدها جملة كاملة: حط قبلها ; أو نقطة، وبعدها فاصلة. لو هي جوه جملة واحدة: حطها بين فاصلتين عادي.

Subtle cases the ACT likes

Double joiners. One joiner is enough. ✗ Although the road was closed, but we walked. ✓ Although the road was closed, we walked. ✓ The road was closed, but we walked.
Semicolon + FANBOYS. ✗ The road was closed; and we walked. Choose either the semicolon or the comma + and, not both.
Which vs it. ✓ …, which surprised everyone. ✗ …, it surprised everyone. Changing one word can turn a correct sentence into a splice.
Long clauses hide splices. In a 30-word sentence, look for a second subject-and-verb after a comma — that is where the splice hides.
Colon vs semicolon. If the second clause explains or proves the first, both are usually fine; if it is just the next event, use a semicolon or period, not a colon.
Three correct, one wrong. In NOT questions, the period, semicolon and comma + FANBOYS versions are all acceptable; the lonely comma (or the missing punctuation) is the answer.

A 3-step method

Find the verbs in the sentence and their subjects. Each subject + finite verb is a clause.
Label each clause: independent (can stand alone) or dependent (starts with because, which, although…).
Check the joint. Two independent clauses → period, semicolon, comma + FANBOYS, or colon if the second explains the first. Independent + dependent → comma or no comma, never a semicolon. Then read the sentence again with your answer in place.

Worked examples

Worked example 1 — the pronoun splice

Passage: The Nile Valley is very narrow in some __places, this is__ why many villages were built right beside the river.

Choose the best version of the underlined portion.

A) NO CHANGE
B) places this is
C) places; this is
D) places, it is

Answer: C

A) ✗ Wrong: Comma splice — "this is why…" is a full independent clause.
B) ✗ Wrong: Run-on — two clauses with no punctuation.
C) ✓ Correct: The semicolon joins two complete, closely related clauses.
D) ✗ Wrong: Still a comma splice, and "it" is less clear than "this".

Worked example 2 — however after a comma

Passage: The museum planned to reopen in __June, however, the__ new roof was not finished until September.

Choose the best version of the underlined portion.

A) NO CHANGE
B) June; however, the
C) June however the
D) June, however the

Answer: B

A) ✗ Wrong: "however" is not a FANBOYS word; two clauses with only commas = comma splice.
B) ✓ Correct: Semicolon before however (two complete clauses), comma after it.
C) ✗ Wrong: Run-on — no punctuation at all.
D) ✗ Wrong: Still a comma splice, and the comma after however is missing.

Worked example 3 — when the second clause explains

Passage: The ferry captain followed one simple __rule, no__ boat left the harbour during a storm.

Choose the best version of the underlined portion.

A) NO CHANGE
B) rule no
C) rule; that no
D) rule: no

Answer: D

A) ✗ Wrong: Comma splice — "no boat left the harbour" is a complete clause.
B) ✗ Wrong: Run-on.
C) ✗ Wrong: "that no boat left…" is a dependent clause, so it cannot follow a semicolon.
D) ✓ Correct: The first clause is complete, and the second explains what the rule was — a colon is perfect.

Worked example 4 — a NOT question

Passage: Laila trained for the marathon for six __months. She finished__ the race in under four hours.

Which of the following alternatives to the underlined portion would NOT be acceptable?

A) months, and she finished
B) months; she finished
C) months, she finished
D) months; as a result, she finished

Answer: C

A) ✓ Acceptable: comma + FANBOYS joins two independent clauses.
B) ✓ Acceptable: a semicolon joins two closely related independent clauses.
C) ✗ The answer: NOT acceptable — a comma alone between two independent clauses is a comma splice.
D) ✓ Acceptable: semicolon before the transition, comma after it; "as a result" is logical.

Classic traps

The pronoun splice: a comma before it / this / they / there that starts a new clause.
The fake joiner: however, therefore, moreover, then after a comma cannot join two clauses.
The semicolon + dependent clause: "; because…", "; which…", "; although…" are always wrong.
The double joiner: although … but, because … so, ; and — one joiner only.
The extra comma before FANBOYS: "She finished the essay, and sent it" — no second subject, no comma.
The period/semicolon twins: if two choices differ only by period vs semicolon, both are equally right or equally wrong — look elsewhere.

Fast strategy for test day

When the choices swap commas, periods and semicolons, cover the joint and ask: "Is the left side a complete sentence? Is the right side?"
Both complete → period, semicolon, comma + FANBOYS, or colon (if it explains). One side dependent → comma or nothing.
Spot however/therefore → check for a semicolon or period before it.
In NOT questions, find the comma splice or the fused sentence — that is your answer.
Read the full sentence before choosing; the second subject may be far from the comma.

السؤال اللي تسأله لنفسك

غطّي علامة الترقيم واسأل: الجزء اللي قبلها جملة كاملة؟ واللي بعدها جملة كاملة؟ لو الاتنين كاملين: نقطة أو ; أو فاصلة + and/but/so أو : لو التانية بتشرح الأولى. لو واحد منهم ناقص (بيبدأ بـ because أو which): فاصلة أو مفيش حاجة، وممنوع ;.

Practice

Two short ACT-style essays, 5 questions each. Every item tests how clauses are joined — sometimes NO CHANGE is correct. Explanations appear after you answer.

Remember

Two independent clauses: period, semicolon, comma + FANBOYS, or colon when the second explains the first.
A comma alone = comma splice; nothing at all = run-on.
however / therefore / then are not FANBOYS: semicolon or period before them when they join two clauses.
A semicolon needs a complete sentence on BOTH sides — never before because, which, although.
One joiner only: no although … but, no ; and.

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