Punctuation Rules and Modifier Placement

Semicolons, Colons, and Commas

Semicolons

A semicolon joins two independent clauses that are closely related:

$\text{Independent clause}\textbf{ ; }\text{independent clause}$

Rule: What comes before AND after the semicolon must each be a complete sentence.

Example: "The experiment yielded unexpected results; the team decided to replicate it before publishing."

SAT Trap: A semicolon before a dependent clause or phrase is always wrong.

• Wrong: "The experiment; which yielded unexpected results."
• Right: "The experiment, which yielded unexpected results, was published."

Colons

A colon introduces an explanation, list, or elaboration. The clause before the colon must be independent.

Example: "The study had one fatal flaw: the sample size was too small."

Rule: Whatever follows the colon amplifies or specifies what came before.

Commas — The Big Three

Rule	Example
Nonrestrictive (extra info) elements	"Dr. Chen, who studies marine biology, published a paper."
Items in a series	"The study measured height, weight, and heart rate."
After introductory elements	"After reviewing the data, the team revised their hypothesis."

Nonrestrictive vs. Restrictive:

• "Students who study regularly perform well." (Restrictive — no commas, defines WHICH students.)
• "My sister, who studies regularly, performs well." (Nonrestrictive — commas, adds info about an already-identified person.)

Modifier Placement

A modifier should be next to the word it describes. A misplaced or dangling modifier creates confusion.

Dangling modifier: "Walking through the museum, the paintings caught her eye."

This says the paintings were walking. Fix: "Walking through the museum, she noticed the paintings."

SAT Pattern: The test often presents a sentence with a modifying phrase at the beginning and asks which subject should follow the comma.

Strategy: After a modifying phrase + comma, the very next noun must be the person or thing being modified.

• "Renowned for its biodiversity, Costa Rica attracts thousands of ecotourists annually." (Costa Rica is renowned — correct.)
• "Renowned for its biodiversity, thousands of ecotourists visit Costa Rica annually." (Ecotourists are renowned? — incorrect.)