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Is a comma needed in following sentence?

The scientist presented his theory, and went on to attack the opponents.

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I think the comma is unnecessary, and the sentence would probably be better without it:

The scientist presented his theory and went on to attack the opponents.

Here's why: According to one grammar site:

In sentences where two independent clauses are joined by connectors such as and, or, but, etc., put a comma at the end of the first clause.

Incorrect: He walked all the way home and he shut the door.
Correct: He walked all the way home, and he shut the door.

However, that same site goes on to say:

If the subject does not appear in front of the second verb, a comma is generally unnecessary.

Therefore, if you had used a pronoun after the conjunction, you would want to keep the comma in:

The scientist presented his theory, and he went on to attack the opponents.


Another writer put it this way:

Two independent clauses – two sentences – separated by a coordinating conjunction – for, an, nor, but, or, yet, so – must also be marked by a comma before the coordinating conjunction.

Sentence A: Correct comma usage: John ran to the store, but he walked home.

Notice that both clauses on either side of the comma can stand alone – they make sense. They also are separated by a coordinating conjunction. They need a comma.

Sentence B: Incorrect comma usage: John ran to the store, but walked home.

Are both clauses able to stand alone? John ran to the store can stand alone. Walked home cannot stand alone. This sentence has two of the elements – one independent clause and the coordinating conjunction – but lacks one of the key elements. Walked home is not an independent clause; therefore, you will not use a comma in a sentence like this one.

A friend of mine (who majored in English) taught me this guidance about 15 years ago after I had asked him to proofread a column I wrote. At first I was skeptical about what he said, but it wasn't hard to find several online writing sites echoing that same advice. It seems to be a sound rule of thumb.


Here is another example:

A compound sentence must have two independent clauses – not just two verbs, two nouns, or two groups of words that are not independent clauses. Look at this example:

Joe read the book and saw the movie.

In the above example, two verb groups are being joined by and. The second verb group does not have a subject; thus, it is not an independent clause. Therefore, no comma belongs before and.

One more:

Use a comma to separate independent clauses in a compound sentence when they are separated by a conjunction.

The comma goes after the first clause and before the coordinating conjunction that separates the clauses.

Make sure they are independent clauses and not some other construction where commas are not required.

Correct: We washed the dog, and then we cleaned up the mess that he made.

This contains two independent clauses with their own subject and verb: We washed and we cleaned. The third clause, that he made, is a subordinate clause, so the rule does not apply.

Incorrect: We washed the dog, and then cleaned up his mess.

There is only one subject. This is a single clause, not two independent clauses. The subject we has a compound verb.

Correct: We washed the dog and then cleaned up his mess.

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Commas are not always needed, but when used they introduce a pause.

If you think a pause there is warranted, go ahead and put one in.

Otherwise leave it out.

(Note: The trend these days is to use commas only when they're needed for clarity.)

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