Present Perfect: Uses

The present perfect is often used for an action that started at some time in the past and is still continuing now. Often, the words for (with a length of time) and since (with a starting time) are used along with the present perfect.

He has lived in Canada for five years.
(He started living in Canada five years ago, and he's still living there now.)

She has worked at the University since 1994.
(She started working at the University in 1994, and she's still working there now.)

Sometimes, it's important to say that something happened (or didn't happen), but it's not important (or not known) when it happened. In this case, we can use the present perfect too. In this case, we often use the words already, yet, ever or never along with the present perfect. These words usually go before the past participle in the sentence.

I've already seen that film. I don't want to see it again.
(It doesn't matter when I saw it.)

Have you ever been to Germany?
(It doesn't matter when you went — I just want to know whether you have been there or not.)

This use is a little more difficult than the other two. In this case, the action happened at some time in the past, but the effect of the action is still important now. It's easiest to understand this use if we compare present perfect sentences with simple past sentences.