Breakdown of D'instinct, l'homme a couru, mais il trébuche sur la branche.
l'homme
the man (m)
il
he
sur
on
mais
but
Questions & Answers about D'instinct, l'homme a couru, mais il trébuche sur la branche.
Why does the sentence suddenly switch from the past tense (a couru) to the present tense (trébuche)?
This is the présent de narration (historical present). The sentence starts in the passé composé to establish that the event happened in the past, but switches to the present tense right at the climax of the action. This makes the storytelling feel suddenly vivid and immediate, as if you are watching it happen right now. While shifting tenses mid-sentence can sound a bit clunky in English, it is a very natural and highly common storytelling device in French.
Could you say par instinct instead of d'instinct?
Actually, yes, both are correct! However, d'instinct is a very common fixed expression that often translates best as the single adverb "instinctively." You will frequently see the preposition de used this way to form adverbial phrases of manner, such as de force for "by force."
Does trébucher always take the preposition sur for tripping "over" something?
Yes, when you want to specify the object that caused the fall, you use trébucher sur (literally "to trip on"). Even though in English we often say we tripped "over" an object, French strictly uses sur, as in trébucher sur une marche (to trip on a step).
If we didn't want to use the historical present for vividness, what tense would trébucher be in?
If we maintained a standard sequence of past tenses, it would be in the passé composé: mais il a trébuché. Because tripping on the branch is a sudden, completed action that interrupts the ongoing situation (running), it requires the passé composé (or the passé simple in formal literature) rather than the imparfait.