What stuctures can I use after "warn"?

Question

"Don't put any personal information online", our IT teacher said.

1. Our IT teacher warned us against putting personal information online.

2. Our IT teacher warned us not to put any personal information online.

But how about the following examples? I have seen "warn"can be followed by a that clause and I don't know if these examples could be right:

3. Our IT teacher warned us that we did not put any personal information online.

4. Our IT teacher warned us that we mustn't put any information online.

Answer 1

You warn someone that something is true which could have a bad effect on them.

The teacher warned us that we had only two more weeks to prepare for the exam.

The policeman warned us that the road ahead was closed because of an accident.

You could say

The teacher warned us that it was not safe to put personal information online.

Answer 2

"Warn" can take a huge number of prepositions. All the following are valid:

• The teacher warned us not to put information online.
• The teacher warned us that we should not put information online.¹
• The teacher warned us that putting information online was not safe.
• The teacher warned us about putting information online.
• The teacher warned us that we put information online.²
• The teacher warned us that we did put information online.³
• The teacher warned us against putting information online.
• The teacher warned us by putting information online⁴.

Notes:

1. Or "must not" or "could not" or "must" etc.
2. i.e. that we put information online but should not have done so. Putting "not" in here makes this a warning the we should put information online, but hadn't done so.
3. "did" can be used for emphasis here.
4. The warning was provided by the information that the teacher put online.