All of these words can be used to show time.

Since can be used with a present perfect tense or a present perfect continuous tense to refer to a point or period of time in the past up to the present.

I have lived here since my childhood.

I have been living here since my childhood.

For can be used with a present perfect or a present perfect continuous tense to refer to a period of time up to Now.

We have lived here for 15 years.

We have been living here for 15 years.

Ago

Ago is used when you date back from Now to a point of time in the past.

It was two years ago that I first met Susie.

His father died two years ago.

Before

Before is used when you date back from any point of time in the past or future.

The roof must be repaired before the rain starts.

Exercise

Fill in the blanks with since, for, ago or before.

1. I haven’t seen him ………………………… last week.

2. He has been working here ……………………… three years now.

3. I saw him in Tokyo two months …………………………..

4. I have seen him …………………………

Answers

1. I haven’t seen him since last week.

2. He has been working here for three years now.

3. I saw him in Tokyo two months ago.

4. I have seen him before.