If you remember this simple rule, you will never be confused again: After a modal verb, you always use only the base form of the verb. (base form = the infinitive form with no "to.")
I may go to Paris next year.
I must study harder.
I could not understand the answer.
I can read a little Chinese with the aid of a dictionary.
I shall call you tonight.
The election will occur on November 6. (I am so nervous!)
It might rain, but it probably won't.
Members at usingenglish.com should be respectful of one another.
I would speak Chinese if I could, but I can't.
Credit: I have given you the 9 main modal verbs, as listed in The Oxford Dictionary of English Grammar.