Which is correct (in the example of..."If I was a liberal, I would....")
If it's true (i.e., that at one time in my life I was a liberal or that I will be a liberal in the future), I'd use the indicative: "if I was"
Indicative: Reference to the past
EX: If I was a liberal way back in 1970--I just can't remember if I was or if I wasn't--then I would have voted for the Democrates. Indicative: Reference to the future
EX: Let's say/If I was a liberal, then I would vote for the Democrates.
If it's false, or a presupposition that's contrary to the fact (i.e., I have never been a liberal, and I don't expect to ever be a liberal, but for the sake of argument, I will suppose that I am, I'd use the subjunctive: "If I were"
EX: I am not a liberal, but if I were a liberal, I wouldn't vote Conservative.
With conditionals (if), the line between the Indicative (i.e., reference to the future) and the Subjunctive can be somewhat fuzzy because both express: let's assume X. But there's a way to separate them: use the indicative when you want to expess something as being true, and use the subjunctive when you want to express something as being false.
Indicative: It's hypothetical, and it's true
EX: If I was to get the job, .... (I am looking into the future; I accepted the job. It's true.)
Subjunctive: It's hypothetical, and it's false
EX: If I were to get the job, ... (I am looking into the future; I accepted the job. It's false.)
There's a famous song lyric by Jewel: "If God was one of us", and people often comment that she should have used 'If God were one of us", but then the subjunctive would have taken away the intended meaning: God understands human nature because God has the ability to take human form (i.e, Jesus).
Originally Posted by American Heritage Dictionary
According to traditional rules, you use the subjunctive to describe an occurrence that you have presupposed to be contrary to fact:
if I were ten years younger, ... if America were still a British Colony, ...
The verb in the main clause of these sentences must then contain the verb would or (less frequently) should:
If I were ten years younger, I would consider entering the marathon. If America were still a British colony, we would all be drinking tea in the afternoon.
When the situation described by the if clause is not presupposed to be false, however, that clause must contain an indicative verb. The form of verb in the main clause will depend on your intended meaning:
If Hamlet was really written by Marlowe, ... If Kevin was out all day, ...
Remember, just because the modal verb would appears in the main clause, this doesn’t mean that the verb in the if clause must be in the subjunctive if the content of that clause is not presupposed to be false:
If I was (not were) to accept their offer, I would have to start the new job on May. He would always call her from the office if he was (not were) going to be late for dinner. 8 Another traditional rule states that you are not supposed to use the subjunctive following verbs such as ask or wonder in if clauses that express indirect questions, even if the content of the question is presumed to be contrary to fact:
We wondered if dinner was (not were) included in the room price. Some of the people we met even asked us if California was (not were) an island.
if clauses—the reality. In practice, of course, many people ignore the rules. In fact, over the last 200 years even well-respected writers have tended to use the indicative was where the traditional rule would require the subjunctive were. A usage such as If I was the only boy in the world may break the rules, but it sounds perfectly natural.