What's the rule(s) for I was vs I were?

Hello. I'm looking for a simple answer to which is correct: "I was" or "I were." I hear both in conversation but don't know if there is a hard-and-fast rule for useage, if it varies according to situation, or what.

My confusion lies in that I usually think of "were" belonging to plural nouns and "was" belonging to singular, especially "they were" and "we were"--obviously, you don't say "they was" or "we was" under any circumstance.

And also, one wouldn't say "I were going shopping when I were hit by a car."

I'm asking because MS Word's grammar checker sometimes insists I use "I were" when I think it should be "I was." In this particular instance, I'd typed "I wish I was God" and it insisted it should be "I wish I were God." (No, I don't have a God complex. But if I was--or I were--God, I'd make English less complicated.)

Thanks in advance for the help.

My advice is to look for subjunctive mood on the Internet.

"subjunctive after wish. Yet another traditional rule requires you to use were rather than was in a contrary-to-fact statement that follows the verb wish: I wish I were (not was) lighter on my feet. Many writers continue to insist on this rule, but the indicative was in such clauses can be found in the works of many well-known writers." (from The American Heritage® Book of English Usage)

In the ordinary past tense of the verb to be, was is the first and third person singular: I was late and were is the second person singular and the plural: You were right. It is not acceptable in standard English to use were for the first and third person singular: I were late, and was for the second person singular and the plural: You was right. In the past subjunctive, however, the situation is more fluid. Historically the past subjunctive of be is were for all persons, singular and plural: I wish she were here; Suppose I were rich; If only they weren't so expensive. It is quite common, though, to use was instead of were for the first and third person singular: I wish she was here; Suppose I was rich. This is perfectly acceptable in colloquial English, but in formal writing it is better to stick to were. Remember that the fixed phrase as it were cannot be changed – never as it was.