Hi Marina. Generally, you can say "sitting in a chair" or "sitting on a chair". However, it depends on the type of chair. You're more likely to "sit in an armchair" than "sit on an armchair". You're always going to "sit on a stool", never "sit in a stool". But for a regular seat like you would use at a dining table, you could "sit in it" or "sit on it".

Helloo~! I found this :) Hope this helps ^^ http://english.stackexchange.com/a/72836

An extract from the Oxford Advanced Learner's Dictionary: the chair [sing.] the position of being in charge of a meeting or committee; the person who holds this position: Who is in the chair today?

If you use 'on' when the chair does not have 'arms' and 'in' when it does, then everybody should understand. If you think about a high-chair for an infant, they are 'enclosed' so that they cannot fall. So you would use 'in'.