1. He is sitting in an armchair [which has armrests].

Question

2. He is sitting on an armchair [which has armrests].
[Can we use both 'in' and 'on' in the sentences?]

3. She is sitting on a stool.
4. She is sitting in a stool.
[Can we use 'in' in this sentence? We use 'on' here, right?]

Answer 1

1. yes
2. not normally, unless he's sitting on an armrest
3. yes
4. not unless she's inside the wood that makes up the stool!