with the Present Perfect Tense
SINCE means 'from some definite POINT in the past till NOW'. It is used with the beginning of an action (since two o'clock, since 1989, since my last birthday)
FOR means 'length of time up to NOW'. It is used for a period of time (for two weeks)
1. Peter has been in England for three weeks. (and he is still
2. John has known his friend since 1985. (and he still knows him)
3. It has rained. (the streets are wet)
4. She has not slept well therefore she looks tired.
All these actions reach up to the present.
The Pres. Perfect Tense is often used with such adverbs as
today, this week, this year, so far (= bisher, bis jetzt), up to now (= bis jetzt), often, always, never, ever, not yet, recently (= lately)
Now try this exercise.