Breakdown of Après avoir quitté le bâtiment, il faut que vous alliez à l'abri.
vous
you (plural/formal)
avoir
to have
à
at / to / in (general locations) / about (what one thinks about / reacts to)
aller
to go
Questions & Answers about Après avoir quitté le bâtiment, il faut que vous alliez à l'abri.
Why is it alliez instead of the regular present tense allez?
The phrase il faut que (it is necessary that / you must) expresses an obligation. In French, this phrase is a classic trigger for the subjunctive mood, which is why you must use the subjunctive form vous alliez.
Wait, alliez looks exactly like the imparfait (imperfect) tense. Is it?
Good eye! It is identical. For most verbs, the nous and vous forms in the present subjunctive look exactly like the imparfait. While aller has a highly irregular subjunctive stem for most pronouns (que j'aille, qu'il aille), it switches back to the regular all- stem for nous and vous, adding the standard -iez ending. So vous alliez happens to be both the imparfait and the present subjunctive form!
Why does the sentence say Après avoir quitté instead of just Après quitter for "After leaving"?
In French, the preposition après must be followed by the past infinitive to say "after doing something". You form this with the infinitive of the helping verb (avoir or être) plus the past participle (quitté). It literally means "After having left". Never use après with a simple infinitive or a present participle.
What exactly does à l'abri mean here?