Breakdown of J'ai pensé que le brouillard disparaîtrait vite.
je
I
avoir
to have
vite
fast, quickly
penser
to think
Questions & Answers about J'ai pensé que le brouillard disparaîtrait vite.
Why is disparaîtrait in the conditional tense ("would disappear") instead of the future tense?
This is the rule for reported speech when the main verb is in the past. If you state the thought in the present, you use the future tense: Je pense que le brouillard disparaîtra (I think the fog will disappear). But because the main verb was moved to the past (J'ai pensé), the future tense disparaîtra must shift back to the conditional disparaîtrait.
The ending -ait in disparaîtrait looks like the imperfect tense. How do I easily spot that it is the conditional?
The conditional tense is formed by taking the future stem (which almost always ends in "r") and adding the imperfect endings. The future stem of disparaître is disparaîtr-. If this were the imperfect tense, it would use the present-tense stem and look like disparaissait.
Could I say Je pensais instead of J'ai pensé for the first part of the sentence?
Yes, absolutely! In fact, Je pensais (imperfect) is often more common for expressing a belief or an ongoing state of mind in the past. J'ai pensé (passé composé) suggests a more sudden thought that crossed your mind at a specific moment. Both past tenses require the second half of the sentence to shift into the conditional.
Why does disparaîtrait have a circumflex accent (^) on the "i"?