Consider this sentence:

There are many questions in this question

  1. I don't know to answer which one
  2. I don't know which one to answer
  3. I don't know which to answer
  4. I don't know which one I should answer

I guess 2 and 4 are the ones that I heard most. However, I'm not sure what makes the first one different from the rest and illegible.

closed as off-topic by Jason Bassford, Davo, Chenmunka, Varun Nair, shin Jan 24 at 5:58

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions asking for someone to find and correct errors or improve the phrasing are considered requests for proofreading and are off-topic. Please edit your question to focus on something in particular that you are unsure about; if that's not possible, see websites for proofreading instead." – Jason Bassford, Davo, Chenmunka, Varun Nair, shin
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 2, 3, and 4 are all fine, but I can't give you a more complete answer without more information about why this is difficult or confusing for you. – Andrew Jan 13 at 23:21
  • @Andrew more details was added – Ahmad Jan 14 at 5:26

These three sentences:

  1. I don't know which one to answer.

  2. I don't know which to answer.

  3. I don't know which one I should answer.

are correct.

The first two are infinitival clauses. Other similar examples, formed by a wh-word and an infinitive, are:

  1. I don't know what to do.

  2. I don't know how to answer this question.

  3. I don't know where to go.

  4. I don't know who to ask.

  5. I don't know when to stop.

Just as 5, 6, 7, 8 and 9 are reduced forms of the sentences containing the embedded questions below:

5'. I don't know what I should do.

6'. I don't know how I should answer this question.

7'. I don't know where I should go.

8'. I don't know who I should ask.

9'. I don't know when I should stop.

2 and 3 are abridged forms of 4.

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