1. The water stays in the glass.

Question

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?]

Answer 1

Yes, in these sentences "in" and "into" are interchangeable.

=)

Answer 2

Thank you. What about the four sentences?

Answer 3

Yes, all the sentences are fine. =)