The dishes in the kitchen is dirty.

Good guess! The subject and the verb don't agree. What's the probable cause for the problem? Kitchen (a singular noun) is right in front of is (a singular verb). If kitchen were the subject, that would be okay. But, it's not. Cross out the prepositional phrase and you're left with:

The dishes in the kitchen is dirty.

"The dishes . . . is dirty?" Sounds wrong, doesn't it? The subject is plural, but the verb is singular. They don't agree. The correct version is:

The dishes in the kitchen are dirty.

Once you know how to look for this problem, it shouldn't be too hard to get rid of it when you proofread your paper.

Reversed sentence order

The normal pattern for English sentences is subject-verb. However, there are a few situations where this order is reversed (like this sentence):

o There are snacks on the laundry-room table.

o Where are they?

o On the table are the goodies!

See how the subject comes after the verb in each of these? If you can remember how to locate subjects and verbs, you shouldn't blunder into mistakes when writing reversed-order sentences.