A present perfect tense is often used in subordinate clauses to show that one thing is completed before another starts. Note that the present perfect tense can be used in the subordinate clause only when the verb in the main clause is in the present or future tense.

I will telephone you after I have seen Peter.

After you have completed this form, give it to the secretary.

We will not accept the offer until they have agreed to our conditions.

I will come back after I have had my lunch.

I understand that you have decided to accept the job.

Instead of the present perfect tense, we can use simple present tense in these sentences. Note that the simple present tense doesn’t put an emphasis on the completion of the action.

I will telephone you after I see Peter.

After you complete this form, give it to the secretary.

We will not accept the offer until they agree to our conditions.

A past perfect tense can be used in the subordinate clause when the verb in the main clause is in the past tense. Here the past perfect tense is used to show that one thing is completed before another starts.

After he had finished his studies, he went to the US.

He wrote his first book after he had returned from Nigeria.