Would + Infinitive or Past Simple?

'Would' has quite a lot of different uses. It's often a kind of past tense version of 'will'. Remember that both 'had' and 'would' can be shorted to 'd. But only 'would' is followed by an infinitive without 'to'.

Is would a simple past?

would is the past tense form of will. Because it is a past tense, it is used: to talk about the past. to talk about hypotheses (when we imagine something)

Is would use for past tense?

Technically, would is the past tense of will, but it is an auxiliary verb that has many uses, some of which even express the present tense.