These three words are sometimes confusing to a lot of English language speakers in deciding the right one to use. They are similar, but they are not the same. There are some clues to help you know the context in which ‘ago’, ‘for’, and ‘since’ are used.

AGO

Ago is used to refer to a particular time in the past. Whenever you use ago, you should use the simple past tense. Ago simply means “prior to now” or “before now”.

Examples:

He renovated his house a year ago. (He renovated his house a year prior to now or a year before now)

The lockdown was imposed about a month ago. (The lockdown was imposed on us about a month prior to now or a month before now)

FOR

For is used to refer to a specific time span or period of time. For instance, 4 weeks, one hour, ten years, two decades, etc. Use since with the present or the past perfect tense.

Examples:

We have been talking for three hours.

Grace has been attending college for seven years.

SINCE

Since is used to express an idea indicating the start of a specified period of time that still continues until the present. You should use the perfect tense whenever you use since.

Examples:

We have been talking on the phone since 1 PM.

George has been absent from school since the beginning of the semester.

Since can also be used to mean because. However, this use of since is formal and may cause ambiguity issues.

Example:

I do not recommend signing any contracts now since the coronavirus pandemic still poses a risk.

The above sentence can be rewritten to be:

I do not recommend signing any contracts now because the coronavirus pandemic still poses a risk.

An example of a situation where the sentence with this use of the word since can be ambiguous is:

He has changed his haircut since he was advised to do so.

This example above could mean two things:

1. He has changed his haircut because he was asked to do so. Or
2. He has changed his haircut from the time he was asked to do so.

It is safer to use because or as in situations like this.

NOTE: The trick is to understand the context before choosing the right word.