Can i say " meri maa theirthy hai"
It's a different tense.
मेरी माँ तैर रही हैं = My mother is swimming - Present continuous tense
मेरी माँ तैरती हैं = My mother swims - Simple present tense
Floating means swimming why they gave me wrong
Floating and swimming are two different actions in English
Y meri instead of mere
मेरी is the feminine form and is used because माँ is feminine.
Hello, so y use hain and not hai ? (hain being more polite?)
Words referring to people older than you are treated as if they were plural in order to be respectful. This is why it's conjugated with the plural form हैं.
You use मेरी both for singular and plural feminine nouns so that remains as it is.