English is a member of the Indo-European group of languages - it includes Germanic languages, that's languages like English and German, Romance languages like Italian and Portuguese, Persian languages and many of the languages of the Indian subcontinent. And these languages all shared a common ancestor - for those of you who are interested, it was called proto-Indo-European. This language divided ideas into things which are real on the one hand, and things which are imaginary on the other.
This difference still exists in Latin languages like Portuguese, where you have indicative tenses to describe real things and subjunctive tenses to describe things which are 'wishes', 'untrue at the moment' or 'imaginary'.
If you translate the two sentences:
'He comes every Tuesday' and 'I hope he comes'
into Portuguese, the form of the verb is probably different - the first is indicative and the second is subjunctive.
English makes the same distinction, but it's much easier as subjunctive forms are usually identical to indicative forms, so you can't tell the difference. Basically 'I was' is indicative -
'I was having dinner when you called' or 'I was happy' - both describe real situations.
'I were' is subjunctive.
'If I were you, I wouldn't go' is not a real situation, as I cannot be you ...
'IfI wereyou, I wouldn't go'.
It's very common to use the 'I were' construction in sentences with 'if' - or conditional sentences. This is correct, and technically speaking:
'If I was you ...' is incorrect, at least in formal speech and writing.
Unfortunately, it is quite common to hear native speakers say it, and it might be that the subjunctive form is gradually disappearing from English.
The subjunctive form is a little more flexible though - we can use inversion with the subjunctive, so instead of saying: 'If I were rich, I would build a new house for my family' - we can say: 'Were I rich, I would build a new house for my family'.
Although some people might say: 'If I was rich, I would build a new house for my family' - I don't think anyone would say: 'Was I rich, I would build a new house for my family'!
This flexibility might mean that 'I were' survives, at least in formal communication. I think that's a good thing!
Mark Shea has been a teacher and teacher trainer for fifteen years. He has taught English and trained teachers extensively in Asia and South America, and is a qualified examiner for the University of Cambridge oral examinations. He is currently working with journalists and is the author of the BBC College of Journalism's online English tutor.