1. Dogs can walk, but can't fly.

  2. Dogs can walk, but they can't fly.

Which sentence is correct? Is the "they" required?

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It's not needed. Totally a personal choice here. Both sound fine and convey the same meaning.

  • 1
    I agree, although for the first statement, I would probably have omitted the comma (while in the second, I would've left it intact). They just seem to flow better that way, at least to me. – Deepak Jul 14 '15 at 1:16
  • 1
    "Dogs can walk, but cannot fly" - sounds even better. – Vladimir Kornea Jul 14 '15 at 1:25
  • I agree with @Deepak in that a comma is not necessary in sentence number one. Generally speaking, shorter sentences do not need a comma before the connecting word (e.g., but, and, or). When the connecting word separates two independent clauses, however, as in the case of your sentence number two, a comma IS needed. Another example: "Dogs can walk, but dogs definitely cannot fly." Don – rhetorician Jul 14 '15 at 1:32

Conjunction reduction permits you to omit repeated words either before or after the conjunction, so both are correct. You may say:

"Dogs can walk [...]"

"Dogs [...] can't fly."

As a matter of style and without further context, between the two options I would probably prefer using the pronoun in the second sentence. I think it makes the second sentence more clearly articulated. This would simply be adjoining two independent clauses with the conjunction, with the word but indicating that the second clause is a further note regarding the same subject:

"Dogs can walk. They can't fly."

"Dogs can walk but they can't fly."

One other option worth considering is that conjunction reduction also allows "Dogs can walk but not fly." It is both more formal and concise to omit the contraction, which is unnecessary since you already have the word "can" before the 'clause' preceding the conjunction "but," and you're continuing where you would have otherwise left off:

"Dogs can walk [...]"

"Dogs can [...] not fly."

Also yes, can not as two separate words is acceptable, albeit less common.

Reference Materials: conjunction-reduction. (n.d.) Collins English DictionaryComplete and Unabridged. (1991, 1994, 1998, 2000, 2003). Retrieved from The Free Dictionary by Farlex

Purdue University's Online Writing Lab: Identifying Independent and independent clauses.

The definition of "But" from The American Dictionary of the English Language by Noah Webster, 1828.

'Cannot' or 'can not' from the Oxford Dictionaries website.

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