Learn French: Je regarde le passant admirant cette vitrine. - I am watching the passerby admiring this shop window.

Instead of using the relative pronoun qui and a conjugated verb, like le passant qui admire (the passerby who is admiring), you can use the present participle admirant. This works exactly like the "-ing" form in English and makes the sentence feel a bit more formal and concise.

Why doesn't admirant need to agree with le passant?

When a present participle ending in -ant is used to replace a verb in a relative clause, it is invariable. That means it never adds an -e or an -s. Even if the sentence were about multiple people (les passants), the word would still be admirant. It only agrees with the noun if it is being used purely as a descriptive adjective.

Wait, isn't passant also a present participle?

Yes! Passant is the present participle of the verb passer (to pass). However, in this sentence, it is being used as a noun (le passant) to mean "the passerby". French frequently turns present participles into nouns for people doing an action, similar to words like "assistant" or "participant" in English.

Why does this sentence use vitrine instead of fenêtre?

While une fenêtre is a standard window you would find in a house or office, une vitrine specifically refers to a shop window or a display case. Since the passerby is "admiring" it from the street, they are looking at a store's window display.