1. forgot
I forgot my homework. (This sentence is correct) 

2. together
Together with my friend. (This sentence is correct. Try to make a complete sentence next time)

3. stayed
I stayed home all night! (This sentence is correct)

 
4. promise
I promise with my friend.

> I promised my friend.. (Please make your sentence complete)

5. prefer
I prefer cola (This sentence is correct. I Or you can also say I prefer cola to tea)

6. flavor
Flavor is maen teast.

> What do you mean in this sentence?

7.careful
Be careful Please. (This sentence is correct. Be careful, please.)

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Hello Kelly!

               Thank you so much for doing your homework, I really appreciate your effort. Keep up the good work, Kelly and teacher is always here for you.

See you in the class. ^^

~Teacher Cassie