Many new-comers to Thailand make the mistake of asking 'How do you say 'Yes' and 'No' in Thai?'
The reality is that there's no such word as 'Yes' as we have it in English. To answer in the affirmative, just repeat the verb back.
ใช่ châi means 'it is'
'ใช่' literally means 'it is' - so if someone asked a question with
'ใช่มั้ย' 'châi mái?'
tagged at the end (or inferred), meaning 'is it (so)?' - then 'ใช่ châi' would be the appropriate 'affirmative verb' response.
But if someone asked a question like 'Would you like a cup of tea?', then responding 'it is' or ใช่ châi wouldn't be appropriate.
I put a clip together a while back going into detail on this point - I thought I'd breathe some new life into it as I heard someone last night continue to use 'chai' for everything, sometimes resulting in confused looks and nobody had the heart at the time to correct him.