Grammar Tip 7: since + date, for + period

Take a look at these sentences. Do they sound Ok to you?

1. I have known Bob now since ten years.
2. I have been friends with Bob for ten years.

Well done if you feel that (2) is good, and something about (1) is not quite right. When describing a period of time which goes from past to present - using present perfect tense - you need to use

since with a date or point in time: I have known Bob since 1996.
for with a period of time: I have known Bob for ten years.

But what about this sentence?

I have known Bob since ten years ago.

ago usually requires the past tense and does not work well with 'since' or 'for': I met Bob ten years ago.

Note: many English dialects use present tense with 'since' and 'for': We are together now for ten years. This is not standard or written English, but it is heard often. Present perfect is the best tense to use in this situation.
Watch out if your first language is Dutch. Present tense is usually OK to use here in Dutch, but does not work in English!

However, there are ways of using present tense without 'since' or 'for' to express time:

My friendship with Bob is ten years old.
Bob and I go back ten years.
My relationship with Bob dates back ten years.