Learn French: Nous avons remarqué que l'incertitude les rendait anxieux. - We noticed that uncertainty made them anxious.

In French, when you want to express that something "makes" a person or thing take on a certain feeling or state, you use rendre followed by an adjective. Since anxieux (anxious) is an adjective, you must use rendre. You use faire when "make" is followed by an action verb, like faire pleurer (to make someone cry).

Why is the first verb in the passé composé (avons remarqué) but the second is in the imperfect (rendait)?

This is a classic combination of past tenses. The act of noticing (avons remarqué) was a specific, completed event in the past. However, the effect the uncertainty had on them—making them anxious—was an ongoing, continuous state at that time, which requires the imperfect tense (rendait).

Does the adjective anxieux have to agree with the pronoun les?

Yes. In the structure rendre + direct object + adjective, the adjective always agrees in gender and number with the direct object. Here, les refers to a plural group, so the adjective must be plural. Because the masculine singular form of anxieux already ends in an -x, the masculine plural stays exactly the same. If les referred to an all-female group, it would become anxieuses.

Why is there an l' before incertitude if we just say "uncertainty" without "the" in English?