Can someone please explain to me the ending options ... hai ... hain.... etc... I think it's hain for plural and respect for father grandfather uncle etc... and hai for singular. But here for mother it's also hain.... and yet sometimes it seems hai can be used??? What are the rules please?
In general, 'hai' is for singular and 'hain' is for plural.
As a way of showing respect to those elder than you or in positions of authority (both male and female), words referring to them are treated as if they were plural. That is why we use 'hain' for them.
However, this is not a grammatical requirement. For example, if it's your grandmother asking you the question 'How is your mother?', she doesn't need to use the respectful plural for your mother (since she is older) and will just say तुम्हारी माँ कैसी है?