Definite vs possessive with clothing

Bonjour Harriet !

You use the definite article when expressing a statement in which the owner of the clothes is obvious in the given context:
"Adrien se leva, le pantalon sur les chevilles." (Adrien got up, his trousers on his ankles.)

These forms are used more often in such structures such as:  "<statement> + "," <description with clothing>"

When you have multiple people mentioned, and you need to specify whose clothes they are, then you will revert to possessive adjectives:
"Elle emprunte mon manteau." (She's borrowing my coat.)

Look at this example as well:
"Je mets mon pantalon." (I put my trousers on.)
->Here you need the possessive, as saying "I put the trousers on." could also be meaningful, and you need to give precisions as to specify the context. Moreover, this is not a description, but an action done on this piece of clothing.

I hope that's helpful!
À bientôt !