1. Can he have killed his wife?
2. Could hee have killed his wife?
3. He can have killed his wife.
4. He could have killed his wife.
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Do you use #1? Then, what is the difference between #1 and #2?
Don't you use #3 in English speaking countries?
2 and 4 are used most commonly.
1 is possible, but not often.
3 is not used.