< 3 буду 3. if you ( not to waste) your time you (to finish) the work earlier. but you chat a lot. 4. if he (to invite) me to his birthday i (to buy) him a nice present. 5. if i (to be) you i (to write) a letter to him.

3)If you do not waste your timeyou will finish the work earlier. But you chat a lot.
4)If he invites me to his birthday I will buy him a nice present.
5)If I am you I will write a letter to him.
So, it is all. Thank you

3. If you hadn't wasted your time you would have finished the work early.
4. If he invites me to his birthday I will buy him a nice present.
5. If I were you I would write a letter to him.