For British English, don’t even worry about the subjunctive at all. American English is more conservative on this point, so speakers of standard American English still automatically use the subjunctive in hypotheticals. For example, I would say “it works as if it were magic.” Note: The meaning is that it is *not* magic. In very casual conversation, other may not bother, but in formal writing (in American English), we need to use the subjunctive where called. You could avoid the issue by saying “it works like magic (or “like a charm”).” In college entrance exams for universities in the US, failure to use the subjunctive properly will result in points off. (And you will be tested on it.)

In North America, we are taught that it is technically correct to use the subjunctive (were) in this context, but many Americans (including me) often use the indicative (was) anyway.

This depends on who answers the question. They are both correct. Your choice with as-if clause depends on whether you want to reflect a true situation (in which case you would choose the past simple) or a counterfactual situation (in which case you would choose the subjunctive).

How can I learn English

Yes both are correct.