Please help

Normally one says "in the past". I cannot at the moment think of any case where "in past" would be used instead (I mean "in past" by itself; obviously one can say "in past years", "in past eras", etc.).

"in past time" is acceptable in certain special cases, such as when talking about verb tenses, in my opinion. It is not a general-purpose substitute for "in the past".

Thanks, so "in past time " is grammatically correct ?

In my sentence shouldn't it be "an action which was done in the past ?If not then please give the reasons.

In my opinion "An action which is done in past time" is acceptable in certain cases, such as when discussing the use of verb tenses. However, generally speaking, "in past time" is unusual. Generally speaking, and if unsure, use "in the past".

Thanks again, what about is or was dilemma ? Please shed some light on it as well.

Normally one uses the past tense when talking about past events, of course. So, one would say an action was done in the past. However, when standing objectively "outside the flow of time", as when analysing the use of language, it is not impossible to say something like "an action which is done in the past".

By the way, in modern English you should not put a space before a question