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Question1: What caused the explosion?
Answer1: The explosion could have been caused by a gas leak. (I think this is correct.)
Answer2: The explosion could be caused by a gas leak. (I think this is incorrect.)

Question2: Who stole the money?
Answer1: It could have been Dan, but I’m not sure. (I think this is correct.)
Answer2: It could be Dan, but I’m not sure. (I think this is incorrect.)

I think this way because we can only use could have (done) to talk about past possibility. Both of the Answer2 do not fit the question. Am I right?

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    Don't get too attached to the idea that Past Tense It could have been Dan is the only "correct" response to a question framed in the Past, such as Who did that? Native speakers don't necessarily think like that, not least because the answer is still currently unknown. It's quite true that if the addressee knew who did it, they'd always reply It was John, never It is John. But note that Who wanted ice-cream? is a perfectly natural question for which I do! is a perfectly natural response. English tenses are relatively flexible.
    – FumbleFingers
    Commented Sep 19, 2022 at 16:39

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The first two examples are definitely not wrong.

They are both talking about possibilities associated with a definite event in the past that actually occurred, and that is the correct way of talking about them.

Your examples are grammatically correct, but have completely different meaning.

"An explosion could be caused by a gas leak," talks about future possibility of non-specific events rather than a specific past event.

"It could be Dan" talks about what he is right now (potentially a thief) rather than a specific act of theft in the past that he may or may not have committed.

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  • I totally agree with you. But I didn't get my answer. So I edited my question for you.
    – joy2020
    Commented Sep 19, 2022 at 17:46
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    In each pair, the second is NOT grammatically incorrect. It's grammatically correct, but doesn't fit the situation. Grammar is about whether a sentence makes sense, and these do. I don't think your question is improved, and I am no longer sure I know what your question is. If your question is whether the second in each of the first two pairs of examples are grammatically correct: they are. If your question is: do they fit the answer, they do not. However, the second part of your question isn't really related. And "It could be Dan" isn't talking about the future, it's talking about the present.
    – a101010
    Commented Sep 19, 2022 at 18:11
  • So if your question is really about the second half, I don't understand what your question is.
    – a101010
    Commented Sep 19, 2022 at 18:16
  • Thanks, I get my answer from your above comment. Btw, I deleted the second half.
    – joy2020
    Commented Sep 20, 2022 at 0:43
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English has plenty of exceptions and nuances, but in general, yes, your understanding is correct.

One major exception from what you describe is that the perfect aspect is used to describe action that occurs before another action. It can, therefore, be used for future events. For example:

Today is Monday, and the item hasn't been sold yet. However, you should check again on Thursday, because it could have been sold by then.

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