For, since, ago and before

A Introduction

Jane: You've been playing that computer game for three hours, Mark.

Mark: Not that long, surely.

Jane: You've been playing since lunch. And we finished lunch at two. That was three hours ago.

FOR

Mark has spent three hours playing a computer game. He's been sitting there for three hours.

SINCE

It was two o'clock when Mark started the game. He's been playing since two o'clock.

AGO

Three hours have passed since Mark and Sarah got up from the lunch table. They finished their lunch three hours ago.

We often use for and since with the present perfect to talk about something continuing up to the present.

FOR

We use for to say how long something has continued.

I've been waiting for forty minutes.

We've known about it for two days.

Melanie has been living here for a year now.

SINCE

We use since to say when something began.

I've been waiting since ten past six.

We've known about it since Monday.

Melanie has been living here since last year.

We can also use for with other tenses.

I'm staying in England for a year.

We swam for quite a long time.

We can often leave out for (but not from some negative sentences).

We've had this car (for) six months.

I haven't seen Vicky for a day or two.

We can use the adverb ago to talk about a past time measured from the present. Six months ago means six months before now.

I passed my driving test six months ago. NOT since six months

Vicky wrote to the company weeks ago.

David first met Melanie a long time ago.

Have you seen Emma? ~ Yes, just a few minutes ago.

We put ago after the phrase of time, NOT ago six months

We use before (not ago) with the past perfect, e.g. had done.

I bought a car in August. I'd passed my driving test three months before. (in May, three months before August)

Vicky finally received a reply to the letter she had written weeks before.