Learn English – Difference between watching a film THROUGH tv and ON tv

In virtually every language, prepositional phrases are the most difficult parts of that language for foreign speakers to learn. Native speakers grow up using idiomatic, prepositional phrases in their native language and accept them as natural and normal.

Sometimes, there is only one preposition that can be used. E.g., my cat is in physical contact with the floor, and is entirely supported by the floor, so my cat is 'on' the floor. There is no other preposition I can use that would meet those conditions.

Sometimes, the selection of a preposition is more arbitrary and difficult to explain. E.g., My brother is on holiday. But, my brother is not in physical contact with 'a holiday', nor is he entirely supported by it, so why do we use the preposition 'on'. No particular reason that I know of, it's just what we say. In French, this would be translated to, 'Mon frère est en vacances', which literally translates to, 'My brother is in holiday'. It sounds odd in English, but perfectly normal in French.

Moving on to your question, the correct way to say this in English is:

You can watch a film on TV. (Note: TV is uppercase.)

Why? Because that is the idiom we use in English. We could have said:

You can watch a film in TV.

You can watch a film through TV.

You can watch a film over TV.

You can watch a film with TV.

Any of these prepositions could have been adopted in English for using a TV to watch movies, but for some reason we selected 'ON' instead. If you use any other prepositional phrase than 'on TV' people will probably know what you mean, but they will think that your English is non-standard.