Breakdown of Il est vital de ralentir à cette profondeur.
être
to be
ce
this (before a noun)
à
at / to / in (general locations) / about (what one thinks about / reacts to)
il
it (impersonal subject)
Questions & Answers about Il est vital de ralentir à cette profondeur.
Why do we use de before ralentir here?
When you use a formal impersonal expression like Il est + adjective (here, vital) followed by an infinitive, you must connect them with the preposition de. This rule applies to most impersonal phrases, such as Il est important de or Il est impossible de.
How could I add "unless stopping completely" to this sentence using the current lesson's topic?
You would add à moins de followed by the infinitive: Il est vital de ralentir à cette profondeur, à moins de s'arrêter complètement. We use à moins de + infinitive rather than à moins que + subjunctive because the implied subject (the person slowing down or stopping) remains the same for both actions.
Why is the preposition à used for "at this depth"?
In French, measurements indicating a specific level, distance, or spatial point—like depth, altitude, or height—take the preposition à. You will see this in similar phrases like à cette hauteur (at this height) or à cent mètres (at one hundred meters).
Can I say C'est vital de ralentir instead of Il est vital de ralentir?