I have to go to the piano class right away.
When I was a child, I was frightened by a spider. Now I'm frightened by a mouse.
I plan to do my homework after a while.
I have lots of food.

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1.
Alice: I have to go to piano right away.
Correct: I have to go to the piano class right away.
Explanation:
*Piano is a musical instrument. It is not a place.

2.
Alice: When I was child, I was frightened by spider. Now I'm frightened by mouse.
Correct: When I was a child, I was frightened by a spider. Now I'm frightened by a mouse.
Explanation:
*Use an article before a noun.

3.
Alice: I plan to do homework after a while.
Correct: I plan to do my homework after a while.
Explanation:
*Be specific in saying that it is your homework, and not somebody else's

4.
Alice: I have lots of food.
Correct: NO CORRECTION

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Hello, Alice! Thank you for submitting your homework. Please read the following corrections and explanations carefully. If you have questions and clarifications, you can ask me in our class. Thank you!