Question

the potato is in your plate- singular
the potatoes are in your plate - plural (why plate doesn't change to plates?)​

Answer 1

because we are talking about many potatoes in a single plate, not in more plates.

Answer 2

Answer:

because e are having many potatoes in one plate.....

so it will not change to plates...

hope it helps you ✔✔☺☺☺☺