Which structure is not correct?

Mehrgan said:

1. One thing you need to do is choose the right dictionary and...
2. One thing you need to do is to choose the right dictionary and...
3. One thing you need to do is choosing the right dictionary and...

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Mehrgan said:

Hi all,
Could anyone please tell me if any of the following sentences is not a normal way of saying what is meant?


1. One thing you need to do is, choose the right dictionary and...
2. One thing you need to do is, to choose the right dictionary and...
3. One thing you need to do is, choosing the right dictionary and...




Thanks a lot in advance.

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fivejedjon said:

No commas,
The first two are fine; I prefer #1. #3 is incorrect.

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Mehrgan said:

Thnaks. How about the following sentence?

Choosing the right dictionary is one/another thing you need to do.:tick:

If it's correct, then would you please explain how the third example in my previous post is not correct? I used to think the third example was a case of gerunds.

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