If the context is crystal clear and, as such, allows no risk of misunderstanding or ambiguity whatsoever, unlike "Paul jumps into the lake (= Paul jumps into the lake from a certain point)" vs. "Paul jumps in the lake (= Paul is in the lake and jumps out)", is it grammatically acceptable in AmE to substitute "in" and "on" for "into" and "onto" to indicate movement or the idea of movement, for all but the most formal prose?
E.g.
Should we go in the pool first, or should we shower first? source
Let's get on the bus before the good seats are taken. source
Let me translate this in the language of non-BS… source
Best Answer
In and into are not, strictly speaking, grammatically interchangeable in formal writing. On and onto have a little more leeway, but are also not always interchangeable.
But, colloquially we often use in for into.
It is perfectly acceptable to say go in the pool, jump in the pool, etc. But, as you correctly stated above, that introduces ambiguity:
You can get on the bus, and you can get in the bus. But, they have subtly different meanings:
But, note that we don't get on the car. We do get in or into the car, though.
For whatever reason buses, trains, planes, and other large conveyances have a notion of being board-able (employing the preposition on), but cars and vans generally do not. (This may be convention.)
Compare this to a bicycle, motorcycle, etc.. You get on a bicycle, but this is the physical sense of being on top of it.
Languages are always translated into one another. Never in. But, you can speak in a language.
Some other cases where on and onto are not interchangeable:
But, they are interchangeable when on carries the sense of on top of.