we use tumhain when we give an order OR advise to someone.... TUMHAIN = you have to. TUMAIN college jana paryga = u have to go to college,, tumhain exercise karni chahey = u should do exercise. in simple form (tumhain = you).

TUMHE : Tumhe mere shache talibilm ho ! ( You are my true student ! ) TUMHEIN : Ye katab tumhein dena thee ! ( I have to give you this book ! )

they are same words تمہیں (tumhein) is the spelling.