Bella: 1.I will buy all of the math thief series.
becaause,they are very funny.
And It's made funny math!
Correct: I will buy the all series of math thief because it's very funny. It's also have funny math activities.
Explanation:
a. The sentence was rephrased.
b. Don't begin your sentence with conjunctions - because, and.

Bella: 2.I will study hard 11years old.
Because,11years old,study it's difficult.
(Let's study hard0
Correct: I will study hard because I'm already 11 years old. Our lessons are very difficult. (Let's study hard).
Explanation:
a. Don't begin a sentence with conjunctions - a word that is used to connect phrases or parts of a sentence.
b. Be careful with your punctuation marks.

Bella: 3. Iwill go to the other countries.
Because,It is interesting.
Correct: I will go to other countries because it's interesting.
Explanation:
a. Be mindful with capitalization of words.

Hello, Bella! Good job for your homework. Very well said. Study the corrections that we have in here. Happy weekend. Take care always! ^^

~ Teacher Belle.