Can someone please explain why the etre is used in the following question instead of Avoir.

Question

I'm a bit confused by this because most of the other sentences do not use je suis. Thanks
Je suis avec ma copine depuis 2005.
I have been with my girlfriend since 2005.

Answer

Hi Melanie, I think I understand your confusion.
Remember that in English, "have" is an auxiliary verb in sentences like "I have been" (this tense is called the present perfect in English, and is composed of a present tense conjugation of "to have" + the past participle of the verb, which in this case is "to be" - this structure is very similar to Le Passé Composé).
Now, when we use "depuis" to express "since" or "for" in French, we use it with the present tense. In constrast, in English, we use the present perfect.
So, "être avec" (to be with someone) in the present tense is "Je suis avec". In the English equivalent, "have" appears because we need to transpose the tense to the present perfect and so it becomes "I have been". You see than "to be" is still there but it's become the past participle in English, and have is just the usual auxiliary verb that's always part of that tense. I hope that makes sense?

Answer

In fact, to my opinion, the proper translation would be 'I am with my girlfrieng since 2005'. This makes more sense.

Answer

That sounds a bit odd, though? Anyway, the point is, if you want to describe the fact that you have been with your girlfriend since 2005 you can never use "j'ai" in French. So while you can say:

"I have been with my girlfriend since 2005"

You need to say:

"Je suis avec ma copine depuis 2005"

and never:

"J'ai été avec..."