Modal verbs are a type of verb that allows the speaker to express things such as advice, obligation, order, suggestions, capacity, request, permission, ability, and likelihood. Two important ones are “can’t have” and “needn’t have”.^[1]https://phoneenglish.es/blog/2017/04/modals-cant-have-and-neednt-have/

While they seem similar they actually express quite different things.

Can’t Have

This modal verb expresses something that you are very sure didn’t happen in the past or when you believe something was impossible in the past.

When using this modal verb, it follows the following formula:

• Can’t have + Past Participle

For example:

• The prisoner can’t have escaped through that window because it is too small.
• The kid can’t
  have been at the movie theater. He was at home all weekend studying for the
  exam.
• Susie can’t
  have said that. She is too nice and wouldn’t say such mean things.

Needn’t Have

“Needn’t have” expresses when an action was done but was most likely a waste of time and unnecessary.

When using this modal verb, it follows the following formula:

• Needn’t have + Past Participle

For example:

• You needn’t
  have cooked so much food. We already ate before we came here.
• She needn’t
  have bought a new car. I was going to give one to her for her birthday.
• We needn’t
  have studied so hard for the test. It ended up being canceled.

Conclusion

Understanding grammar rules is just the first step — the real progress happens when you practice using them in real conversation. A private lesson is the fastest way to get there.