We use the past subjunctive form here. "Were" in this case because we are imagining being in Tom's position. We use a subjunctive when we are expressing something we want to happen, or an imagined situation.

We use 'Were' instead of 'Was' when we imagine something. For example, If I were you, I would not do that (imagination, but not real). If I were Joy, I would not do that mistake (imagination, but not real).

For an 'if' sentance, 'was' or 'were' could be used. 'Were' is used in this sentance, because it is the subjunctive. The suggestion, to be 'in Tom's position' is presumed impossible. If it were a possibility, 'was' would be used.

Yes, it is because of 'if' . It's a second conditional form, the same as 'I would help him if I had time'. We use 'if' with the past form to talk about an imaginary situation. And yes, you could use 'was'. 'Were' is more correct, but many people say 'was' in this situation, and this is acceptable.

Some examples using "was" IN DIALOG (the writer doesn't think its correct, but wants to show how real people really talk): "Well, Ben Rogers, if I was as ignorant as you I wouldn't let on."--Mark Twain, "Huckleberry Finn" "Well," said Joe..., "if I was yourself, Pip, I wouldn't. No, I would not."--Charles Dickens, "Great Expectations" Some examples using "was" where a famous writer is using it himself (he thinks it's perfectly correct): "Some of my friends spoke as if I was coming to the woods on purpose to freeze myself."--Henry David Thoreau, "Walden" "I foresaw that if I was driven into either of those currents, I should be carried a great way out to sea."--Daniel Defoe, "Robinson Crusoe" "Yet it was evident that if I was to flourish matches with my hands I should have to abandon my firewood."--H. G. Wells, "The TIme Machine" Some examples using "were," the strictly correct usage: "Could I see it from the mountains/If I were as tall as they?"--Emily Dickinson "putting me before the fire as if I were going to be cooked"--Charles Dickens, "Great Expectations" again. Unlike Jo, the voice of the narrator in the book uses "were." And finally, here is H. L. Mencken bellyaching about the death of the subjunctive in 1921: "All signs of the subjunctive, indeed, seem to be disappearing from vulgar American. One never hears “if I were you,” but always “if I was you” ... Perhaps in another generation the subjunctive forms will have ceased to exist."