Learn Portuguese (Portugal): A tigela está muito suja. Não a uses!

In Portuguese, negative words like Não act like magnets that pull the object pronoun to sit right before the verb. If this were a positive command telling someone to use it, the pronoun would go after the verb with a hyphen: Usa-a!

Why does the verb end in -es (uses) instead of -a (usa)?

This is a negative command for tu (informal "you"). In Portuguese, you cannot use the normal command form when telling someone not to do something. Instead, negative commands for tu swap their vowels. For an -ar verb like usar, the ending changes to -es.

What exactly does the a in Não a uses refer to?

It means "it", replacing the noun a tigela (the bowl) so we don't have to repeat the word. Because tigela is a feminine word, we must use the feminine pronoun a. If we were talking about a masculine object like o garfo (the fork), it would be Não o uses!

Why do we use está instead of é for "is" in the first sentence?

We use está (from the verb estar) because being dirty is a temporary state, not a permanent characteristic of the bowl. Once you wash it, it will be clean again!