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Grammar and Usage Unit 2: Sentence structure

A complete sentence needs a subject and a verb and must express a complete thought. Fixing fragments and run-ons is one of the most tested skills.

Covers: Subjects and predicates, clauses, fragments, run-ons and comma splices, simple, compound, complex sentences

Study guide

Every complete sentence has a subject (who or what the sentence is about) and a predicate (what the subject does or is, built around the verb). The simple subject is the main noun or pronoun; the complete subject includes its modifiers. Commands have an understood subject, you.

A clause is a group of words with a subject and a verb. An independent clause expresses a complete thought and can stand alone as a sentence. A dependent (subordinate) clause can't stand alone because it starts with a word like because, although, when, if or which.

A fragment is an incomplete sentence: it's missing a subject or verb, or it's a dependent clause by itself ("Because it rained."). Fix it by attaching it to an independent clause or adding what's missing.

A run-on (fused sentence) joins two independent clauses with no punctuation, and a comma splice joins them with only a comma. Fix either one with a period, a semicolon, a comma plus a coordinating conjunction (FANBOYS), or by making one clause dependent with a subordinating conjunction.

Sentences are classified by their clauses. A simple sentence has one independent clause. A compound sentence has two or more independent clauses joined by a coordinating conjunction or semicolon. A complex sentence has one independent clause and at least one dependent clause. A compound-complex sentence has two independent clauses and at least one dependent clause. Mixing sentence types makes writing more varied and lets you show how ideas relate.

Words like however, therefore, moreover, consequently and otherwise are conjunctive adverbs, not conjunctions, so they can't join two independent clauses with only a comma. Use a semicolon before them and a comma after: "The road was icy; however, the bus arrived on time." When a dependent clause comes first, put a comma after it: "Because it rained, the game was canceled." When it comes after the independent clause, you usually don't need a comma: "The game was canceled because it rained."

Fragments also include -ing or to- phrases standing alone ("Hoping to win the prize.") and relative clauses standing alone ("Which surprised everyone."). Attach them to the sentence before, or add a subject and a finite verb.

Key ideas

• An independent clause can stand alone; a dependent clause can't.
• A fragment is missing a subject, verb or complete thought.
• A comma splice joins two independent clauses with only a comma.
• Fix run-ons with a period, a semicolon, or a comma + FANBOYS.
• Complex sentences join an independent clause with a dependent clause (because, although, when).

Formulas and rules

Common mistakes

• Using a comma alone between two sentences.
• Starting with "Because…" and never finishing the thought.

Words to know

Independent clause

A clause that can stand alone as a sentence.

Fragment

An incomplete sentence.

Comma splice

Two independent clauses joined by only a comma.

Run-on sentence

Independent clauses joined without proper punctuation; fix with a period, semicolon, or comma + conjunction.

Complex sentence

An independent clause plus at least one dependent clause.

Fixing a comma splice

Use a period, a semicolon, a comma + FANBOYS, or make one clause dependent.

Subordinating conjunction

Starts a dependent clause: because, although, since, when, if.

Subject

Who or what the sentence is about.

Predicate

What the subject does or is.

Simple subject

The main noun or pronoun of the subject.

Understood subject

"You" in a command: (You) Close the door.

Dependent clause

Has a subject and verb but can't stand alone.

Clause

A group of words with a subject and a verb.

Simple sentence

One independent clause.

Compound sentence

Two or more independent clauses joined.

Compound-complex sentence

Two independent clauses plus at least one dependent clause.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.