# Would (have) + Had/If (were/would/would have)

I've looked through various sources and can't really understand which are correct and which are not:

1. I would do it had I been him.
2. I would have done it had I been him.
3. I would do it if I were him.
4. I would have done it if I were him.
5. I would do it would I be him.
6. I would have done it would I be him.
7. I would do it would I have been him.
8. I would have done it would I have been him.
9. I would do it if I would be him.
10. I would have done it if I would be him.
11. I would do it if I would have been him.
12. I would have done it if I would have been him.
• Do you have any sources for those to share? For example with any of the bits with "would I" and "if I would". Commented Dec 8, 2016 at 14:13
• @MorganFR It's just that most conditional examples are interrogative and it's hard to understand how they work else-way. Commented Dec 8, 2016 at 14:19
• "if I were him" works with all your examples. 2 is also correct but the rest of the past ones and all the "would I" and the "if I would" seem wrong. That's why I'm asking for a source. Commented Dec 8, 2016 at 14:24

These are all variants on the conditional mood. As a general rule, the "would" statement is known as the "conditional perfect", and can only be used in the clause that describes the consequence, not the one that sets up the condition (i.e. "If I would" is always incorrect).

A short breakdown of the examples, to the best of my knowledge:

1 is incorrect due to mixed tenses. "I would do" is present tense; "had I been" is past.

2 is correct: the tenses match (both past). The removal of "if" and the swapping of the order to "had I" in the second clause is a conditional inversion.

3 is also correct: Here, "were" is being used in a form known as the "present unreal conditional".

4: Correct; "were" is also used for the past unreal conditional (so again, the tenses match)

5&6: both incorrect; "would I be him" would only work in the form of a question, and not as part of the conditional statement: "If I changed into his clothes, would I be him?"

7&8: Same as in 5&6, only framed in the past tense (so the start would be "If I had changed[...]")

9-12 are all incorrect as the conditional perfect ("would[...]") cannot be used in an "if" clause, as mentioned at the start.