I would have come if only you had rung me. This is correct, but it means something else: You just had to call me, and I would have come. ) or in a modified version of your interpretation: I wish you had rung me, then I would have come.
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PastsimpleAnswer to the second question: definitely which. What doesn't make sense here at all.
InchoateknowledgePastsimpleAnswer to the second question: definitely which. What doesn't make sense here at all.
What I must do is write a letter (correct) = Write a letter is what I must do (correct) = I could have come, what I wanted so much.
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