知识点 5

Use commas to set off nonessential clauses/phrases/appositives.

Set off nonessential information with commas, and leave essential information unpunctuated.

核心知识

If removing a clause, phrase, or appositive doesn't change the core meaning of the sentence, set it off with commas. If the information is essential to identifying the subject, leave the commas out.

深入理解

This rule comes down to one question: does the reader need this information to know what you're talking about?

Nonessential (use commas):
→ "Dr. Elena Vasquez, who has studied coral reefs for twenty years, published her findings last month."
Remove "who has studied coral reefs for twenty years" and the sentence still makes sense—we already know which person (Dr. Elena Vasquez).

Essential (no commas):
→ "The researcher who has studied coral reefs for twenty years published her findings last month."
Remove the clause and we no longer know which researcher. The clause is essential.

Appositives work the same way:
→ Nonessential: "Marie Curie, the first woman to win a Nobel Prize, conducted pioneering research on radioactivity."
→ Essential: "The physicist Marie Curie conducted pioneering research on radioactivity."

The test: cover the phrase with your hand. If the sentence still clearly identifies its subject, the phrase is nonessential and needs commas on both sides.

分步讲解

  1. Find the clause, phrase, or appositive in question.
  2. Mentally remove it from the sentence.
  3. Ask: does the sentence still clearly identify who or what it's about?
  4. If yes → nonessential → set off with commas on both sides.
  5. If no → essential → no commas.

常见误解

  • "'Which' always needs a comma before it." "Which" usually introduces nonessential clauses (comma needed), but the real test is whether the clause is removable, not which word introduces it.
  • "Appositives always take commas." Only nonessential ones. "My brother Tom" (if you have multiple brothers) uses the appositive essentially—no commas.
  • "You can use one comma for a nonessential element in the middle of a sentence." You need commas on both sides. One comma creates a mismatched boundary.
题目

示例解析

The Voyager 1 spacecraft ______ has been traveling through interstellar space since 2012, continues to transmit scientific data back to Earth despite being more than 15 billion miles from the Sun.

Which choice completes the text so that it conforms to the conventions of Standard English?

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