They're both in the subjunctive mood. The first one is sometimes called the "future subjunctive" in English and the second one is usually called the past subjunctive:
- "If Tom were to do my homework, I would watch a film."
("were to do" is the future subjunctive of the verb "to do")
- "If Tom did my homework, I would watch a film."
("did" is the past subjunctive of the verb "to do")
I am probably going to get some flak from the modern grammarians on this forum for my use of terms like "past subjunctive" for "did" in no. 2 and God forbid I even utter the term "future subjunctive" because many modern grammarians don't believe that English has a "future indicative" let alone a "future subjunctive", but guess what, boys? I just uttered it. Now, I shall start with the ever-so-slight difference between the two statements at hand:
"If Tom were to do my homework, I would watch a film."
"If Tom did my homework, I would watch a film."
No. 1 uses the future subjunctive of the verb "to do". The future subjunctive is very formal and the rule for it has exceptions, so what I tell you here is going to be how it's used 99% of the time rather than all of the time. For the other 1%, it is used politely to talk about a condition in the future, whether that condition be counterfactual or not. Anyway, the future subjunctive appears a lot in formal English essays, papers, or dissertations, but it is heard in conversation here and there as well. It is used to talk about an event in the future that is "very" unlikely, if not impossible, to occur.
Unlike No. 2, which uses the past subjunctive, No. 1 could be talking about a condition that is counterfactual in the distant future; no. 2 does not talk about the distant future; no. 2 is using the past subjunctive, which talks about a condition that is counterfactual either in the present or near future. For instance:
"If I were to visit the planet Pluto, I would pack a lot of warm clothes."
In my imaginary world, one day I shall visit Pluto and I will have to pack warm clothes.
"If I were to be crowned King of England, I would rule with an iron fist."
In my imaginary world, one day I shall be crowned king and I will rule with an iron fist.
"If one were to look closely at the two examples above, he could easily see the subtlety of the future subjunctive."
Remember that 1% I told you about; here it is. This is an example of politeness using the future subjunctive and it is often found in formal essays and papers. I have written this before in many formal essays in high school and college over the years. I am politely trying to direct the reader's attention to something. It's more polite than the imperative,
"Look closely at the two examples."
If I were to have used the past subjunctive "looked" in the situation above, it wouldn't sound so polite as it does with the future subjunctive (This is an example of the future perfect subjunctive, which expresses courteousness herein). Usually, we use the past subjunctive in counterfactual conditions, but it can be used for politeness in some cases like the future subjunctive; however, the future subjunctive is far more polite and its meaning is clearer. For instance, last week, my grandfather and I were talking about his getting eye surgery to correct his blindness in his one bad eye and I said,
"That would be great if you had your sight back."
(past subjunctive use of politeness)
However, I could have said this a nimiety of ways:
"That would be great if you [should] have your sight back."
(A mixture of past subjunctive "would" with present subjunctive "[should] have": a little less polite than the above one)
"That will be great if you [should] have your sight back."
(archaic present subjunctive: not so polite [it could read "if he have"])
"That would be great if you were to have your sight back."
(future subjunctive: very polite)
We know that "were + infinitive" equals the future subjunctive because there is no present subjunctive possibility; that is, one cannot say or write "be + infinitive".
Here's a paradigm of the verb "to be" in the subjunctive so that you can see some of the different forms although I don't list every possible schema because passive voices in the subjunctive are possible in some instances, not to mention continuous constructions.
"If that be the case, I shall eat my hat."
"Be that the case, I shall eat my hat."
(present subjunctive [archaic]: Modern English "is" or "should be" [inversion used in second example])
"If that should be the case, I shall eat my hat."
"Should that be the case, I shall eat my hat."
(present subjunctive replaced by using "should" before the verb [inversion used in second example])
"If that were the case, I should eat my hat."
"Were that the case, I should eat my hat."
(past subjunctive [inversion used in second example])
"If that were to be the case, I should eat my hat."
"Were that to be the case, I should eat my hat."
(future subjunctive [inversion used in second example])
"If that had been the case, I should have eaten my hat."
"Had that been the case, I should have eaten my hat."
(past perfect subjunctive [inversion used in second example])
It is essential that that have been the case at some point.
(present perfect subjunctive: [I show it like this because it is so rare in "if clauses" even in archaic English])
"If that were to have been the case, I should have eaten my hat."
"Were that to have been the case, I should have eaten my hat."
(future perfect subjunctive: [equivalent to the past perfect subjunctive in meaning, but more emphasis on the counterfactual reality of the past situation; inversion used in second example])
I hope that might have helped out those who have been struggling with this difficult concept. In the end, there's very little difference between the future subjunctive and past subjunctive, especially in the original example above. In the original example above regarding Tom and his homework, I would say no. 1 is more unlikely or more polite, depending on the context clues, than no. 2 is.