Use Present Perfect for the Unfinished Past:
The present perfect can be confusing for some, but it is one of the most important rules of grammar. When people talk about things that have already happened but consider the time in which they occurred to be unfinished, they use the third form of the verb with a helping verb. The helping verb for the present perfect is the present tense conjugation of "to have."
I have drunk three cups of Lapsang Souchong tea today.
Mary's hyperactive cur dog has bitten me three times so far.
Mary has walked her hyperactive poodle 100 times this week.
Unfortunately, the only way to know the third forms of verbs is to remember them.
