Before

A) before (adverb)
1) ' at any time before now/ then'
We can use before to mean 'at any time before now'. In British English, a present perfect tense is normally used:
I think I've seen this film before.
Have you ever been here before?

Before can also mean 'at any time before then - before the past moment that we are talking about'. In this case a past perfect tense is used:
She realized that she had seen him before.

2) counting back from a past time
We also use before after a time expression to 'count back' from a past moment - to say how much earlier something else had happened. A past perfect tense is normally used:
When I went back to the town that I had left eight years before, everything was different, (*not ... that I had left before eight years...)

To count back from the present, we use ago, not before:
I left school four years ago. (*not ... four years before / before four years)
3) before, before that and first
Before is not generally used to mean 'before that' or 'first':
I want to get married one day. But before that / first, I want to travel.

B) before (conjunction)
clause + before + clause before + clause, + clause

1) position of before-clause
The conjunction before joins one clause to another. Before and its clause can come either after or before the other clause, depending on what is to be stressed:
I always feed the cat before I have breakfast.
Before I have breakfast, I always feed the cat.
(The meaning is similar: the speaker feeds the cat and then has breakfast. Note the comma in the second structure.)

He did military service before he went to university.
(He did military service first.)
Before he did military service, he went to university. (He went to university first.)

2) present tense with future meaning
In a clause with before, we use a present tense if the meaning is future:
I'll telephone you before I come, (*not ... before I will come.)

3) perfect tenses
In clauses with before, we often use present perfect and past perfect tenses to emphasise the idea of completion:
You can't go home before I've signed the letters.
(= ... before the moment when I have completed the letters.)
He went out before I had finished my sentence.
(= ... before the moment when I had completed my sentence.)
(Note that in sentences like the last, a past perfect tense can refer to a time later than the action of the main verb. This is unusual.

4) before...ing
In a formal style, we often use the structure before.. .ing.
Please put out all lights before leaving the office.
Before beginning the book, she spent five years on research.

C) before (preposition) and in front of

before: time
in front of: place
Compare:
I must move my car before nine o'clock.
It's parked in front of the post office, (*not ... before the post office.)

Before is normally used to refer to time, but it can refer to place in a few cases:
a) to talk about the order in which people or things come in queues, lists, written documents, etc:
Do you mind? I was before/in front of you!
Her name comes before mine in the alphabet.
We use 'a' before a consonant and 'an' before a vowel.

b) to mean 'in the presence of (somebody important)':
I came up before the magistrates for dangerous driving last week.

c) in the expressions right before one's eyes, before one's very eyes