Why 'were'?

because it is in the subjunctive mood

If you go, let me know. [not subjunctive]

It's a conditional clause, signalled by "if", and it takes the subjunctive form of the verb. Or it used to; I think Osso is right to say it's going out of fashion (but not with me, not yet.)

Subjunctive
The subjunctive is a special, relatively rare verb form in English.

Structure of the Subjunctive
The structure of the subjunctive is extremely simple. For all verbs except the past tense of be, the subjunctive is the same as the bare infinitive (infinitive without "to"):

be (past) be (present) all other verbs (past & present)
I were
you were
he, she, it were
we were
you were
they were I be
you be
he, she, it be
we be
you be
they be I work
you work
he, she, it work
we work
you work
they work

The subjunctive does not change according to person (I, you, he etc).Use of the Subjunctive
We use subjunctives mainly when talking about events that are not certain to happen. For example, we use the subjunctive when talking about events that somebody:

•wants to happen
•hopes will happen
•imagines happening
Look at these examples:

•The President requests that you be present at the meeting.
•It is vital that you be present at the meeting.
•If you were at the meeting, the President would be happy.
The subjunctive is typically used after two structures:

•the verbs: ask, command, demand, insist, propose, recommend, request, suggest + that
•the expressions: it is desirable, essential, important, necessary, vital + that
Here are some examples with the subjunctive:

•The manager insists that the car park be locked at night.
•The board of directors recommended that he join the company.
•It is essential that we vote as soon as possible.
•It was necessary that every student submit his essay by the weekend.
Notice that in these structures the subjunctive is always the same. It does not matter whether the sentence is past or present. Look at these examples:

•Present: The President requests that they stop the occupation.
•Past: The President requested that they stop the occupation.
•Present: It is essential that she be present.
•Past: It was essential that she be present.
The use of the subjunctive as above is more common in American English than in English, where should + infinitive is often used:
•The manager insists that the car park should be locked at night.
•It was essential that we should vote as soon as possible.
We usually use the subjunctive were instead of "was" after if (and other words with similar meaning). Look at these sentences:

•If I were you, I would ask her.
•Suppose she were here. What would you say?
Why do we say "I were", "he were"?
We sometimes hear things like "if I were you, I would go" or "if he were here, he would tell you". Normally, the past tense of the verb "to be" is: I was, he was. But the if I were you structure does not use the past simple tense of the verb "to be". It uses the past subjunctive of the verb "to be". In the following examples, you can see that we often use the subjunctive form were instead of "was" after:

•if
•as if
•wish
•suppose
Formal

(The were form is correct at all times.) Informal

(The was form is possible in informal, familiar conversation.)
If I were younger, I would go. If I was younger, I would go.
If he weren't so mean, he would buy one for me. If he wasn't so mean, he would buy one for me.
I wish I weren't so slow! I wish I wasn't so slow!
I wish it were longer. I wish it was longer.
It's not as if I were ugly. It's not as if I was ugly.
She acts as if she were Queen. She acts as if she was Queen.
If I were you, I should tell her. Note: We do not normally say "if I was you", even in familiar conversation.

Some fixed expressions use the subjunctive. Here are some examples:

•Long live the King!
•God bless America!
•Heaven forbid!
•Be that as it may, he still wants to see her.
•Come what may, I will never forget you.
•We are all citizens of the world, as it were.
Now check your understanding »

were, of course.