Past: I could do that Past Participle: I could have done that Future: I could do that Examples: Yesterday, I could walk to the store. I could have bought milk. Tomorrow, I could bring money to buy milk.

The verb in the phrase, "I can do that" is "to do" and has nothing to do with the verb "to be (able)" (able is an adjective); therefore, future is "I will do that.", past tense is, "I did that.", and conditional is, "I could do that."

Do you have a specific sentence?