"It must had been"

English as a Second Language English Vocabulary, Grammar and Idioms
1

Is “it must had been” correct usage? “It must have been” sounds better to me but what if I’m talking about the past which, it turn, refers to the past. For example:

This morning, the creek was running especially fast. It must had been the rain overnight.

Thanks

2

‘must had been’ is an impossible structure. Use ‘aux + have been’ even for regression:

John: ‘It snowed last night. You must have been cold.’
John said that it had snowed the previous night and that I must have been cold.