grammar

??Why cant 'clean' cannot be in past tense?

It's a bare infinitive there, not a present-tense form.

??Why 'clean' cannot be in past tense?

Note the correct way to ask this question: Why can't "clean" be in the past tense?

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Thanks for the correction. But, I dont understand why we use root verb in the sentences " He helped me clean my room."

The verb that follows "help" will be the "bare infinitive" -- you don't make that change tense.

You make the change to the verb "help":

Will you help me clean?
He helps clean my house every week.
He helped clean my car last week too.
He has helped clean your house too.
He was helping her clean her basement when they found the treasure.
Etc.

??:

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Thanks for the correction. But, I dont understand why we use root verb in the sentences " He helped me clean my room."

He helped me clean my room = He helped me to clean my room.

The "to" is optional. It's not needed.

BarbaraPAThe "to" is optional. It's not needed.

I agree that "to" is optional, but it would be clearer to the OP why "clean" is the verb needed, not "cleaned".

??Why 'clean' cannot be in past tense?

It's part of a catenative structure, of which English has hundreds. A catenative structure is a two-verb structure. The form of the second verb is governed by the first verb. Only the first verb has a tense (helps, helped, has helped).

The verb "help" governs the base form. help me clean