"I will give this book to you tomorrow."
Translation:मैं यह किताब तुम्हें कल दूँगी।
ईस is the oblique case. Like 'him/us' in english. The oblique case is when the verb/action is happen to/from/by/etc something.
In the above sentence, the ''you''/तूझे is the something that needs to be in the oblique case, because the action ''give'' is happen to ''you''.
Hope it is now clearer. My terms in English grammar are not my cup of tea.