Grammar question, please help.?

Where are my pants? ( 'are' is used for plural nouns...people, places, things)
Where are my swim trunks? (same as above)

Whenever you're iffy about sentences like those, try inverting the sentences. For instance, the first sentence, "Where are my pants?" would read, inverted, "My pants are where?" If that sounds right, then that's the correct usage!

Would the inverted form of "What is my pants" be grammatically correct? It would be, "My pants is what?" -- that doesn't sound right to me.

You can do this for many different sentences and usually easily determine the correct usage of verbs.

Hope I helped!

Where are my pants.
where are my swimming trunks.

Where are my pants? and Where my swim trunks? are correct.

The plural pants and trunks require ARE. Where is my shirt? Where are my shoes? Here is your shirt. Here are your shoes.