When do we use for, since, ago and before?

• Mom: You have been playing that computer game for five hours, Tim.
• Tim: No, mom. Not that long, surely.
• Mom: You have been playing since lunch. And we finished lunch at one. That was five hours ago.

We use 'for' and 'since' with the present perfect.

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 you for two hours.
• We've known about the class for two days.

Michelle has been living here for a year now.

SINCE:

We use 'since' to say when something began.

• I've been waiting since ten past seven.
• We've known about it since Monday.
• Michelle has been living here since last year.

We can also use 'for' with other tenses.

• I'm staying in Greece for a year.
• We played for a long time.

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

• We've had this house (for) seven months.
• I haven't seen Dennis for a day or two.

AGO WITH THE PAST:

We can use the adverb 'ago' to talk about a past time measured from the present.

• Eight months ago means eight months before now.
• I passed my driving test four months ago NOT since four months.
• Emma wrote a letter to the authorities weeks ago.
• David first met Clair a long time ago.

BEFORE WITH THE PAST PERFECT:

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 four months before.
• Emma finally received a reply to the letter she had written weeks before.