Learn English – Difference between “across” and “through”

Prepositions can be tricky because they often have multiple meanings depending on context.

"By" can mean "near", as in, "The house is by the water." It can also mean "using", as in, "We travelled by car." That works here. "We shared by email" means "we shared using email".

"Through" usually means "passing over the boundaries of", as in, "We drove through Spain". It carries the implication of going in and coming back out -- you don't use "through" when you stop inside the place or thing. You might use "into".

We sometimes use "through" to describe a method of communication, I guess on the thinking that the message travels "through" this communication system. Like, "I sent a letter through the mail", as in, the message travelled through the postal system. Or, "We kept in contact through the telephone." So in that sense you can say, "We shared through email", i.e. the sharing went through the email system.

Note this usage is pretty much limited to communication. You wouldn't say, "We travelled through train", you'd say "We travelled by train." ("We travelled through a train" would mean that you walked up and down the length of the train, or that you walked in one door of a train and then out the opposite door, not that the train carried you somewhere.)

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.