For, since and ago are all prepositions that can be used for time. When talking about time, they each cover specific times, usually to demonstrate the duration of an action or event. This article explains the difference between them, and when they can and can’t be used together – with an exercise to practice. 

Using “for”

For is used for duration – it describes how long something happens for, independent of its start and finish. It be used with any tense, and is placed before a unit of time.

• I have been learning English for five years.
• He was waiting for two hours.
• They had been living in France for a month.

In all these examples, the duration (how long for) is a unit of time.

Using “ago”

Ago is used to show duration by telling us when something happened in the past, and is placed after a unit of time that measures the time between the past action and now. It is usually in the past tense, marking a past event.

• I drank a coffee ten minutes ago.
• We were watching TV two hours ago.

Using “since”

Since is used to describe duration by telling us when something began. It tells us a duration by showing the start, but does not express a unit of time on its own. It is almost always used with the present perfect or present perfect continuous, as it shows an action that started in the past and is still happening now. It is usually placed before a specific point in time.

• I have been learning English since 1987.
• She has been a private tutor since last year.

Since and ago can sometimes be combined, when we want to use since before a unit of time, instead of a specific starting point.

• I have been listening since ten minutes ago.

For, since and ago exercise

Choose for, since or ago to complete the following sentences.

1. We have been waiting here ____ hours.
2. I wanted to leave ten minutes ____.
3. He has been reading that book ____ last week.
4. I came to Brighton three years ____.
5. She has been cycling ____ an hour.
6. They have been learning English ____ two weeks ____.
7. Jim was in the kitchen ____ the whole afternoon.
8. ____ I moved to the seaside, I have swum in the sea a dozen times.
9. This town was a lot prettier 30 years ____.
10. They have been working on the new house ____ fifteen months.

Answers:

Choose for, since or ago to complete the following sentences.

1. We have been waiting here for hours.
2. I wanted to leave ten minutes ago.
3. He has been reading that book since last week.
4. I came to Brighton three years ago.
5. She has been cycling for an hour.
6. They have been learning English since two weeks ago. (or just for)
7. Jim was in the kitchen for the whole afternoon.
8. Since I moved to the seaside, I have swum in the sea a dozen times.
9. This town was a lot prettier 30 years ago.
10. They have been working on the new house for fifteen months.