Learn French: J'ai vu la poussière quand l'équipe a démoli le toit. - I saw the dust when the team demolished the roof.

The English translation says demolished, which is a completed action, so we use the passé composé a démoli. If the sentence meant you saw the dust while they were in the middle of demolishing the roof, you would use the progressive aspect était en train de démolir or the imperfect tense démolissait.

l'équipe means a team of several people. Why is the verb a démoli in the singular form?

Even though a team is made up of multiple people, the word l'équipe is a singular feminine noun in French. You must treat it like a singular unit and use the singular verb form, which is why it is l'équipe a and never l'équipe ont.

Why is the past participle démoli spelled with an i at the end?

démolir is a regular verb ending in -ir. To form the past participle for the passé composé, you drop the -ir and add -i. This follows the exact same pattern as finir becoming fini or choisir becoming choisi.

Could I use lorsque instead of quand here?

Yes, lorsque and quand both mean when and can be used to connect these two past events. lorsque is just slightly more formal, but both are perfectly correct in this context.