No single part of speech gives writers more grief than the mighty verb. Think about all the elements you must take into consideration when forming verbs. They have tense, number, person, voice, and mood. Where things get especially dicey is with a verb’s mood, in particular, the subjunctive mood.
Take a look at this sentence:
I wish I was/were vacationing on a tropical island instead of at my desk working.
The correct verb choice is the second one: were. It expresses the subjunctive mood, something we use to convey a wish or a condition that isn’t true. And since the speaker isn’t actually vacationing on a tropical island, this is a perfect case for using the subjunctive mood.
Most writers will intuitively plunk in some form of a past-tense version of the verb “to be” in a sentence like our example. The important thing to remember about the subjunctive mood is to choose the correct version. When using the subjunctive mood, the correct version is “were.”
One good test is to mentally add “but I’m not” to the sentence. If that makes it a true statement, then it’s a likely candidate for the subjunctive mood:
I wish I were vacationing on a tropical island instead of at my desk working (but I’m not).
If he were ten feet tall (but he’s not), he could wash the windows without using a ladder.
You might hear that the subjunctive mood is fading from common use, and that’s probably true (Maeve wrote about that on The irrealis “were”). However, it’s still a hallmark of correct usage, and savvy writers will try hard to get it right. Tevye, from Fiddler on the Roof, got it right in the song “If I Were a Rich Man.” So did Bobby Darin in “If I Were a Carpenter.” It might be one of the worst songs of all time, but the grammar is spot-on.