"Grandmother studies a lot."

Translation: दादी बहुत पढ़ती हैं।

[hectoryake]

Why I use हैं instead of है?

[vinay92]

As a way of showing respect, words referring to people older than you (such as दादी) are treated as if they were plural. So, it is conjugated with हैं, which is the plural form of the verb.