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Hi,

In short sentences, no comma is required. Example: Either you're with me or you're against me.

But in longish sentences, are commas required? Example: Either he didn't know what John was capable of, or he didn't care enough about the consequences.

Is a comma after 'capable of' required because it's a longish sentence?

Thanks.

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A comma signifies a brief pause in speaking or reading.

In sentences like those it's optional. A comma after a long first part gives the listener a chance to digest what the writer has just said.

In short sentences the listener doesn't need this moment to understand,. But the speaker may wish to make the second part of the sentence more forceful.

eg Either you stop or I'll shoot.

eg Either you stop, OR I'll SHOOT.

Clive

Comments  
anonymousIn short sentences, no comma is required. Example: Either you're with me or you're against me.

Right. This is an exception to the rule. The two clauses also have to be parallel grammatically, as they are in your example. This is done for the reader. The comma is too heavy for such a light thought.

anonymousBut in longish sentences, are commas required? Example: Either he didn't know what John was capable of, or he didn't care enough about the consequences.

Yes. Every time. There are several reasons for this, the most important of which is to signal to the reader that the conjunction is coordinating. Also, it is a matter of style convention. If you always punctuate the same way, the reader can relax and stop bird-dogging commas.

anonymousIs a comma after 'capable of' required because it's a longish sentence?

See above. The short sentence is the rare exception.