A duck is swimming on the water.

I believe part of the duck is in the water.

A duck is swimming in the water.

She admire to the chicken.

Admire something.
{She} is the third person singular, so please add -s at the end of {admire}.

She admires the chicken.

The cat is despise to it cat food.

We say {despise something}.
{The cat} is the third person singular, so please add -s at the end of {despise}.
Please distinguish {it} and {its}.

The cat despises its cat food.