Write an email to my teacher saying i won't be able to finish the homework on time, be polite
[Insert the honorific and name of your teacher here],

I am sending you this email to let you know I will not be able to finish the homework you assigned in time. I still intend to finish it, and I am planning to send it to you as instructed originally by [insert a realistic completion date here] at the latest.

Please let me know if this is an appropriate alternative, if you would like to propose another date, or if there is another alternative to make up for my completing this homework on time.

Thanks in advance for your understanding,
[Insert your name here]
Thank you
You are most welcome. Please let me know if there is anything else I can help you with.