"If I was..." or "If I were..."

A common question is whether 'If I was ...' or 'If I were ...' is correct. The answer is that they are both correct, but in different contexts. 

If I were = Imaginary

If I were is used to express an imaginary situation in the present.

If I were you, I would take a vacation.
I would give the test again if I were the teacher. 

If I was = when I was

If I was is used to express something that was true when or if something else happened.

If I was late for class, I got into trouble.
I had to do extra homework, If I was absent.

Unreal / Second Conditional

The second conditional, or unreal conditional is formed in the following manner:

If + Subject + Simple Past + Objects, Subject + Would + Verb + Objects

If I had more time, I would take up a new hobby.
If Jane moved to New York, she would get an apartment in Brooklyn.

Remember that it is also possible to end the sentence with an 'if' clause. In that case, do not use a comma to separate the two clauses.

Subject + Would + Verb + Objects If + Subject + Simple Past + Objects

The school would hire more teachers if the district voted for a tax increase.
The economy would improve if the people paid less for health insurance.

 Verb 'Be' - Unreal / Second Conditional

In the case of the verb 'be', the unreal conditional takes the conjunctive form of the verb 'were' for all subjects. In other words, in the 2nd conditional use 'were' for I, he, she, and it, as well as other subjects.
If Susan were the director, she would invest in new marketing strategies.
I would invest in the stock market if I were a rich man.
The conjugation of the verb 'be' as 'were' indicates that these are imaginary situations.

Reality Check

You may have often heard people use 'If I was ...' in the same sense. The fact is that native speakers use the incorrect form so frequently that it is becoming standard usage. It's interesting to note that Cambridge University accepts 'If I was ...' for the unreal conditional on their English learning test series whereas ETS (English Testing Service) does not. This is a case of descriptive grammar (how the language is used) winning out over prescriptive grammar (how the language should be used). It certainly causes many headaches for English learners!

Past Conditional: If = When

There is a case in which 'If I was ...' (or 'If she / he / it was...') is correct. This form is used to indicate an action that occurred in the past if a given situation arose. In this case, the meaning is similar to 'when'. Notice how this usage refers to a past situation that was true in a specific circumstance, rather than referring to an imaginary situation in the present.

If + Subject + Simple Past + Objects, Subject + Simple Past + Objects
OR
Subject + Simple Past + Objects If + Subject + Simple Past + Objects
I was always in a rush when I was young. If I was in a hurry, I usually forgot my lunch.
She visited Tom if she was in New York when she lived in the United States.
When I was in a hurry, I always forgot my lunch.
Jennifer used to live in the United States. She visited Tom if she was in New York.

In this case, 'if' is acting as a time clause to discuss what happened in a certain situation. Compare this usage to the first conditional, which can use the simple present in both clauses.

If he comes to town, we go out for lunch. = When he comes to town, we go out for lunch.
If I was available, we spent some time chatting. = When I was available (in the past), we spent some time chatting.

To improve your understanding of conditionals, this conditional forms page reviews each of the four forms in detail. To practice conditional form structure, this real and unreal conditional form worksheet provides a quick review and practice exercises, the past conditional worksheet focuses on using the form in the past. Teachers can use this guide on how to teach conditionals, as well as this conditional forms lesson plan to introduce and practice the first and second conditional forms in class.

Source: http://esl.about.com/od/common_mistakes/a/If-I-Was-Or-If-I-Were.htm?utm_content=20160426&utm_medium=email&utm_source=exp_nl&utm_campaign=list_esl&utm_term=list_esl