Breakdown of Les cerceaux que j'ai vu lancer sont tombés dans l'eau.
je
I
être
to be
l'eau
the water
dans
in, inside
Questions & Answers about Les cerceaux que j'ai vu lancer sont tombés dans l'eau.
Why is it j'ai vu and not j'ai vus, since les cerceaux comes before the verb?
This is the exact exception for verbs of perception (like voir or entendre) followed by an infinitive. Normally, the past participle agrees with a preceding direct object. But here, the hoops (les cerceaux) aren't doing the throwing (lancer); they are being thrown. Because they are the object of the infinitive rather than its subject, vu does not agree and remains invariable.
When would vu actually agree before an infinitive?
It agrees only if the preceding direct object is the one doing the action of the infinitive. For example, if you saw dogs running: Les chiens que j'ai vus courir. Because the dogs are the ones doing the running, vus gets an s to agree with them.
Why does tombés get an s later in the sentence?
Why is lancer in the active infinitive form instead of a past participle like lancés to mean thrown?