I have to study.
Because I want to be a doctor. so I must be smart.
==> I have to study because I want to be a doctor, so I have to be smart.

I have to sleep early.
Because I want to be taller than now.
==> I have to sleep early because I want to get taller.

I have to read the book.
Because my father said,"A book will help your study." so I must read the book.
==> I have to read books because my father said that a book will help me with my studies, so I must do it.

I have to eat breakfast,lunch, and dinner.
Because If I don't eat the food, I'm so tired.
==> I have to eat my meals because I feel tired when I don't eat.

I have to go to school every day.
Because I am a student.
==> I have to go to school everyday because I am a student.

Hello, Andrew! Thank you so much for doing your homework. You did a good job in constructing long sentences. Kindly check the corrections that I made for you. See you on Friday~!