The word ‘unless’ expresses a negative connotation and expresses a strict condition. It replaces ‘if not’; therefore is always followed by a positive verb. 

It is used as a linking word in conditional sentences, which means unless one thing happens, another thing cannot happen.

 

Let’s consider the following example:

We cannot go to college if we don’t finish high school.

In the above sentence, the negative condition is ‘if we don’t finish high school’.

 

So instead of using ‘if we don’t’, you can use ‘unless’ and rewrite the sentences follows,

We cannot go to college unless we finish high school.

 

Another way to write the above sentence would be;

If we don’t finish high school, we cannot go to college.

 

Using ‘unless’ it can be written as

Unless we finish high school, we cannot go to college.

 

Using unless in conditional sentences:

 

- First Conditional sentences:

Unless + present tense

Example: 

We cannot go to college unless we finish high school.

You cannot expect an appraisal unless you work hard. 

 

- Second conditional sentences:

Unless + past tense

Example:

We wouldn’t go to college unless we finished high school.

You wouldn’t expect an appraisal unless you worked hard.

 

- Third conditional sentences:

Unless + past perfect

Example:

We wouldn’t have gone to college unless we had finished high school.

You wouldn’t have expected an appraisal unless you had worked hard.

 

NOTE:

‘Unless’ cannot be used for the situations and things that did not happen.

 

Let’s consider the above examples, which are

We wouldn’t have gone to college unless we had finished high school.

You wouldn’t have expected an appraisal unless you had worked hard.

 

The above sentence may seem grammatically correct but does not sound natural. 

So it is advised not to use ‘unless’ in ‘third conditional sentences’. 

 

So you can see how ‘unless’ is used to convey negative conditions and replace ‘if not’ in a sentence. This makes the sentence sound better. In order to get a better grip on this topic, try to make some sentences of your own by imagining and observing the situations around you and solve some exercises to get better understanding.