Can you explain the difference?
I thought you were asleep.
I thought you'd be asleep.
I thought you were sleeping.

Question

Answer

They all basically mean the same thing.

The first uses 'asleep'  as an adjective.  'I believed,  as a fact,  that you were in a state of sleeping'

The second is much the same,  but it contains an unstated condition.  (I assumed,  given what I knew that you would be in a state of sleep)

The third uses sleep as a verb.  'I believed, as a fact,  that you were committing the act of sleeping.'