The most common use of the word 'were' is straightforward and should not cause problems. It is the simple past tense of the word 'are' (from the verb 'to be'), and as such is used along with the pronouns 'you', 'we' and 'they', as in these examples:

You were out when I came to see you. We were too tired to watch the film last night. They were disappointed with the results.

'Were' is also used with these same pronouns to form the past continuous tense, in which it is followed by the present participle ('ing' form), for example:

You were studying every evening during the winter. We were hoping to stay for another two weeks. They were travelling by train through Europe.

In the above instances, when using the pronouns 'I', 'he', 'she' or 'it', we would use 'was' instead of 'were'. Here are one or two example sentences:

I was irritated by the noise of the traffic. He was always ready to listen to me.

She was running for a bus. It was raining all last night.

However, in some cases it is correct to use 'were' instead of 'was', for example in hypothetical clauses introduced by 'if':

If I were able to, I would take a year off to travel. She would come with us if she were feeling better.

The same rule applies after the verb 'to wish':

I wish I were free to do as I like. He wishes he were here with us.

In spoken English, however, 'was' is frequently used instead of 'were' after 'if' and 'wish'.

We also sometimes use the phrase 'as it were', meaning 'so to speak'.

'Were' should not of course be confused with the contracted form 'we're', which stands for 'we are':

He knows we're expecting him. We're always ready to help them.

Occasionally I have seen 'were' substituted for 'where' in written English, and vice versa. 'Where' is a completely different word that refers to place or position:

They asked where the nearest bank was. Where are you going tomorrow morning?

Since 'were', 'we're' and 'where' all have distinct meanings and different pronunciation, it ought to be easy to distinguish between them. Perhaps the use of 'were' after 'if' and 'wish' is not one that immediately springs to mind for some, but students of English as a foreign language who wish to impress could certainly do so by mastering this usage of 'were'.