The subjunctive, “were”, is more typical in formal or written contexts, especially in the US (where many speakers use it even in casual conversation). The indicative, “was”, is officially wrong, but often used informally, especially in British English, where it’s actually rather rare to hear the subjunctive nowadays. You may want to check out my mini article on UK versus US grammar: https://www.italki.com/discussion/170637

Since these sentences are conditional, using "if" to state the premise, the simple past form of the verb (in this case "were") is the correct verb form.

Hi Daniel, You should use "if I were" only when you talk about something hypothetical in the present or in future time and "if I was" is when you talk about something that could be true in the past. Therefore, in both of your example cases, you should say "If I were' since you used "would". If you want to use "was" you should then say it like this: If she was single, I would have asked her out!! Hope that helps ;) Morteza

As Phil said, in UK English although ‘were’ is the grammatically correct form more often than not in speech people use ‘was’ instead, in phrases of this sort.