In general the easiest way to clarify that something happened (in the past) and the action has been completed is by adding the
了 (le) particle after the verb within the sentence.
You can say 我买一本书 wo mai yi ben shu = I buy a book
for the past just add the 了 after "buy" : 我买了一本书 wo maile yi ben shu = I bought one book
Note that in many cases though you might need to guess that the action was in the past because of other indications within the sentence.