Before as an adverb

As an adverb before means ‘at any time before now”. In British
English, before is commonly used with a present perfect tense.

I have seen him before.
Have you been here before?

Before can also mean ‘at any time before the past moment we are talking
about’. In this case it is used with a past perfect tense.

Suddenly I realized that I had seen him before.

Before can also be used with a time expression to count back from a past
moment.

When I went back to the village that I had left twenty years before
everything seemed so different.

Notes

To count back from a present moment, we use ago.

I left that city ten years ago. (= It has been ten years since I left that
city.)

Before as a conjunction

The conjunction before joins one clause to another. Before and its clause
can come before or after the main clause.

I always feed the dogs before I have dinner.
Before I have dinner, I always feed the dogs.

Note the use of the comma in the second structure.

In before clauses we use a present tense to refer to the future.

I will call you before I go. (NOT I will call you before I will go.)

In clauses with before, perfect tenses are often used to emphasize the idea
of completion.

You can’t watch TV before you have finished your homework.
He lost his temper before I had said a word.

In a formal style, the structure before …ing is often used.

You can’t go home before finishing the work.