Difference – A Tin of Biscuits vs A Biscuit Tin

An American English speaker here. A can is cylindrical. A tin is usually rectangular, lower than it is wide (rather flat), and almost exclusively used for containing preserved fish, e.g. "a tin of sardines" and "a tin of kippered herring".

Also, we very specifically use the phrase cookie tin to refer to a squat cylindrical metal box of particular dimensions for selling cookies.

I'm not sure this feat is reproducible from computers in other locales, but when I (in Boston, MA, USA) put "a tin of" (in quotes like that) into google image search, most of the first hits are open tins of fish, a few open metal boxes of Spam (hmm, yes, if I ever had occasion to discuss Spam, I suppose I would refer to its container as a tin and not a can because to me can means round and Spam is in a rectangular box), and a round box of cookies.

I don't know if this actually answers what might be your real question: what does the French government think the difference between can and tin is. That may be something else entirely. Governments use words their own ways.

There is little or no difference in meaning between the two sentences. There is a tiny difference in emphasis, which is hard to explain. It has to do with the way you clarify or remove ambiguity in English by adding redundancy, also illustrated here.

Hypothetical possibility with present tense

In both sentences, "his doing my homework" is a hypothetical possibility. A consequence of that possibility, indicated by would, is that the speaker would watch a film.

English provides a variety of ways to indicate a hypothetical possibility. The simplest is just to precede the clause describing the possibility with the word if. You could say:

If he does my homework, I would watch a film.

However, in this form, the meaning of the present tense is not clear. Does it mean "right now"? No, that's not how the present tense is normally used in English. Does it mean "in general"—that is, "if he often, frequently, usually does my homework…" or "if it is his responsibility to do my homework…"? This is how the present tense is most commonly used in English, but it's probably not the intended meaning in this sentence, since "I would watch a film" suggests that the speaker is talking about only one occasion. However, would can also mean a habitual, recurring action. The little English grammatical words tend to be very ambiguous!

So, in English, we often add clarity by repeating or echoing the same meaning in multiple ways. This helps reduce the ambiguity of each individual word or grammatical choice, especially regarding what the speaker means about the time of an event, whether the event is real or hypothetical, what the consequence is and when it occurs, whether the hypothesis is normal or unusual or an offer in a negotiation, etc.

Past tense to indicate future possibility

To avoid the ambiguity described above, it's common in English to describe a hypothetical future possibility by putting it into the past tense, like this:

If he did my homework, I would watch a film.

Another typical example:

If you visited me next July, we could watch the fireworks together.

You might think that using the past tense to indicate a hypothetical future possibility is ripe for confusion, and you would be right. For example, "If he did my homework, I would watch a film" could also mean a past possibility and a present consequence: if he already finished doing my homework, then I would watch a film right now.

So, when using the past tense to indicate a future possibility, people often add additional words to reinforce the interpretation that the sentence is about the future. For example, the sentence with "visited" has the adverbial phrase "next July", which makes fully clear that that the sentence is about a future possibility.

Subjunctive mood

The sentence about homework does not have an adverbial phrase to make the time explicit. So, to be perfectly clear that you are using the past tense for a future possibility, you might use the subjunctive mood. The subjunctive mood means hypothetical possibility! The subjunctive mood is rare in English, and often the form of the verb doesn't unambiguously indicate the subjunctive mood, but with the verb "to be" and subject "I", the subjunctive mood is completely unambiguous, since "were" can't make grammatical sense in any other interpretation:

If he were to do my homework, I would watch a film.

Infinitive verb form

The reason for the infinitive to do is because the infinitive form of a verb avoids attaching it to a particular time. (For more about that, see this answer.) The past subjunctive were, combined with if, adds somewhat more redundancy to indicate that the sentence is talking about a hypothetical future possibility and its consequence.

Conclusion

Because the sentence talks about homework and watching a film, it sounds like the speaker is referring to hypothetical events that would occur this evening. So, you don't really need the subjunctive mood to be clear about what time is being talked about. The sentence with did is clear enough.

Using the infinitive to do instead of the finite did creates a slightly more abstracted feeling about the possibility. Using the past subjunctive were where it's not necessary adds to that feeling of abstract possibility. As I said, the difference in meaning is just a tiny shade of emphasis. But you can learn a lot about English grammar by understanding why the switch from did to were to do creates that tiny shade of difference!