1. The water stays in the glass.
2. The water stays inside the glass.
3. The water remains in the glass.
4. The water remains inside the glass.
[Can we use all the sentences?]
5. Put salt in one glass.
6. Put salt into one glass.
[Can we use both prepositions? Which one is commonly used?]
Yes, in these sentences "in" and "into" are interchangeable.
=)
Thank you. What about the four sentences?
Yes, all the sentences are fine. =)
Yes, you can use all the sentences to convey the same meaning, which is describing the water not leaving the glass. The variations in the sentences are mostly a matter of personal preference or style.
Regarding sentences 5 and 6, both prepositions can be used interchangeably in this context. Both "in" and "into" can convey the action of placing or adding salt to the glass.