“She's trying on a pair of pants.”
You're shopping for clothes with your sister and her boyfriend. He was walking around the store and has just found you again. He asks where your sister is, so this is your answer.
She's trying on a pair of pants.
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try on (a piece of clothing)
To "try on" clothes means to put them on in order to see how they look on you. You "try on" clothes when you are shopping. You can also "try on" your clothes at home when you want to decide what to wear.
You say "try on ___" with a noun or noun phrase:
She's trying on a pair of pants.
When you replace the noun with "this", "these", "it", "them", and so on, you say "try ___ on":
Did you try them on?
a pair of pants
Even though pants are one single item of clothing, they have two leg parts. Because of this, we always say "pants" with an "-s" at the end, and if you want to count them, you call them "a pair":
Why do you have so many pairs of pants?
You also call these clothing items "a pair":
a pair of glasses
a pair of shorts
a pair of socks