Use commas to separate introductory/dependent elements from independent clauses when needed.
Put a comma after an introductory dependent clause or phrase before the main clause.
核心知识
When a dependent clause, participial phrase, or other introductory element comes before the main clause, a comma marks the boundary between them. Skip the comma, and readers may stumble.
深入理解
English sentences often front-load context before the main idea:
→ "After the hurricane destroyed the coastal wetlands, conservation groups launched a restoration project."
That comma after "wetlands" isn't decorative—it tells the reader: the setup is over, here comes the main clause.
When to use the comma:
After an introductory dependent clause (starts with because, although, when, if, after, while, etc.)
→ "Although the sample size was small, the results were statistically significant."After a long introductory prepositional phrase
→ "In the first comprehensive study of its kind, researchers mapped the entire genome."After a participial phrase
→ "Reviewing decades of climate data, the team identified a clear warming trend."
When the comma disappears:
When the dependent clause comes after the independent clause, you usually don't need a comma:
→ "The results were statistically significant although the sample size was small."
The SAT tests whether you can spot this boundary and punctuate it correctly.
分步讲解
- Look for a dependent clause or phrase that appears before the main clause.
- Common signals: subordinating conjunctions (because, although, when, if, while, after) or participial verbs (-ing, -ed) at the start.
- Place a comma at the end of the introductory element, right before the independent clause begins.
- If the dependent clause comes after the independent clause, a comma is usually unnecessary.
- Check that the independent clause that follows is truly complete (has its own subject and verb).
常见误解
- "Every sentence with 'because' needs a comma." Only when the because-clause comes first. "She left because it was late" needs no comma. "Because it was late, she left" does.
- "Short introductory phrases don't need commas." Very short ones (one or two words like "Today" or "Here") can sometimes skip the comma, but on the SAT, if the comma is offered and prevents misreading, it's the right choice.
- "A comma always goes before 'which.'" Not necessarily—but introductory elements before the main clause do take commas, and that's the rule being tested here.
示例解析
While analyzing sediment cores extracted from the floor of Lake Malawi in southeastern ______ evidence of dramatic climate shifts that occurred in the region over the past 1.5 million years.
Which choice completes the text so that it conforms to the conventions of Standard English?
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