Prepositions and time: FOR vs. SINCE

Use for to introduce durations like 1 second, 3.8 billion years, a long time and a little while.

Life on Earth has existed for about 3.8 billion years.
I listened to a Lady Gaga song for a little while.

Since is used to introduce the point in time when something became true and must always be used with a form of the perfective have.

I have lived on Earth since 1975.
I have been listening to Lady Gaga since 3 o'clock.

Some common mistakes involving since:

WRONG: I have lived here since 30 years. [You can't use a duration with since]
WRONG: I am living here since 1981. [You must use since with a form of have]

See the tag prepositions for related posts.