Breakdown of Tu le savais, tu aurais dû m'avertir avant le krach.
tu
you (informal singular)
avoir
to have
devoir
to have to / must
le
him, it (masculine direct object pronoun)
Questions & Answers about Tu le savais, tu aurais dû m'avertir avant le krach.
How does tu aurais dû translate to "you should have"?
It uses the past conditional (le conditionnel passé) of the verb devoir (to have to / must). Literally, it translates to "you would have had to." In French, this specific tense of devoir is the standard way to express a reproach or to say what someone "should have" done in the past.
Why is avertir in the infinitive form here?
In French, when verbs are chained together, the second verb usually stays in its infinitive (dictionary) form. The phrase aurais dû acts as the conjugated part, so the main action verb avertir (to warn) remains unconjugated.
Why does the pronoun m' go before avertir instead of before aurais?
Object pronouns must go immediately before the specific verb they logically belong to. Because the action is "to warn me", the pronoun m' (me) attaches directly to the infinitive avertir. If there were no infinitive, it would go before the auxiliary (like in the passé composé: tu m'as averti), but the infinitive always "steals" its own object pronoun.
Why is savoir in the imperfect tense (tu savais) rather than the passé composé?
The imperfect tense (l'imparfait) describes ongoing states or background situations in the past. The person possessed this knowledge over a continuous period of time before the crash. If you used the passé composé (tu as su), it would change the meaning to a sudden action, like "you found out" or "you realized."