would vs would have done in this context

Question

Here's the context: I talked to my friend yesterday and this morning I didn't check my messages. Later I tell him:

I didn't think you would reply that fast.

or

I didn't think you would have replied that fast.

Can someone explain clearly what is the difference? My teacher makes it complicated for me.

Thank you in advance.

Answer 1

I didn't think you would reply that fast.

This sentence means future action in the past.

I didn't think you would have replied that fast.

And this sentence means future completed action in past.