Explain the usage of “ was and were” in English as per its grammar rules?

(was) Past simple tense (singular except ‘you’)

(1st and3rd person singular)

• I was at the mall yesterday.
• He was seen at the station.
• She was writing her essay.
• It was quite a catastrophe.

(were) Past simple tense  

(1st and3rd  person plural )

• They were eating.
• We were singing.

(were (with 'you'))

• You (singular) were at the gym.
• You (plural) were (all) at the party.
• Where were you? (used a subsidiary to a question word)
• Were you aware? (Used as a question word)
• If I were you, I would complete my assignment. (suggestion/advice)

We use you to refer to the listener or reader. It is both the subject and the object form. You can refer to one person or more than one person. It is usually clear from the context whether you is singular or plural

'You' always takes 'were' when using the past tense, just like it always takes 'are' in the present tense. These are the past simple forms of the verb ‘to be’. Generally, ‘was’ is used with singular pronouns (one subject), and ‘were’ is used with plural pronouns (more than one subject), but the pronoun ‘you’ is an exception!

WAS is usually used with the pronouns ‘I’, ‘she’, ‘he’, and ‘it’.WERE is usually used with pronouns ‘you’, ‘we’, and ‘they’.

Clearly, both forms I was and I were persist in popular usage. Here is the proof!

In 1964, when Sheldon Harnick wrote the lyrics for the musical Fiddler on the Roof, he had the poverty-stricken Russian milkman Tevye sing “If I were a rich man.”

In 1992, affluent rock star Bon Jovi sang “If I was yours,” but then in 2008, Beyoncé sang “If I were your beloved.”

If you curious to see how the two constructions compare in the world of pop music, I searched a site called ReverbNation. According to the search results, “If I Was” and” If I Were” as song titles are tied at “over 500 songs” each.

Morover, According to linguist Geoffrey Pullum, co-author of the Cambridge Grammar of the English Language (CGEL), there’s no significant difference between using was or were in what the CGEL calls “the irrealis form of the copula.” (A copula is what linguists call a word that links subject and predicate. Irrealis is unreal.)

In Pullum’s view, both “if you was” and “if you were” mean the same thing in such a statement.

Scott Fitzgerald used both forms for statements of unreality. Here are two:

I wish I were twenty-two again … — F. Scott Fitzgerald, letter, 27 Dec. 1925.

… if I was Vassar, I wouldn’t take you … — F. Scott Fitzgerald, letter, 18 Apr. 1938

 On conclusion,

Clearly there is a choice to be made here, and if Fitzgerald could use either form, I was and I were so can others. Unfortunately, not everyone will agree with this explanation because so many people, “I wish I was a rich man” is not standard usage.

There are contexts in which “if I was” can be justified For example, “If she was in love, no wonder she won't left him gone.” In a statement that does not describe reality, or the possibility of reality, however other view that were is still the better choice, if only because a great many employers, clients, and customers still regard “if I was you, I won't insult her for being sensitive” as nonstandard usage.

First comes first. You can't learn the use of was/were if you don't know how to use am/is/are.

The verb "to be" is used differently, depending on the pronoun and the time of the verb you are using: 

Present tense:

I am - You are - She is - He is - It is - We are - You are - They are

Past tense:

I was - You were - She was - He was - It was - We were - You were - They were

Shukram Ghada!

Both are used in denoting the past, with 'was' constituting singular and 'were' constituting plural.

Thanks for invitation....I endorse Mr. Mohammed Ashraf's answer

I am happy all the answers by experts are so good, and I have nothing to add, except that 'were' has one more use, that is, to mean 'if'

1. Were you to leave early, just inform me.

But I am a little surprised at one of the examples given by Ms. Lubaina:

If she was in love, no wonder she won't left him gone.

The reason is, after 'wont' is used there's no possibility of the use of second form of the verb. Look for example,

2. I wont do it again, I promise.

3. They will come tomorrow, but I wont come.

So, according to correct usage, the example sentence by Lubaina would be:

4. If she was in love, no wonder she won't have him gone. or

5. If she was in love, no wonder she won't let him go.

But I may be wrong. I shall be glad to hear the opinions of the experts.

Agree with Experts, Lubaina explained nicely 

I, he, she it  - was

They, we, you - were