Learn French: J'avais vite trouvé mon canif. - I had quickly found my pocketknife.

In compound tenses like the plus-que-parfait used here (avais trouvé), short and common adverbs like vite, bien, or déjà generally go between the auxiliary verb (avais) and the past participle (trouvé).

The current lesson is about pronominal verbs. How would the meaning shift if this sentence used se trouver instead?

While trouver here means to physically find an object, the pronominal se trouver takes on the idiomatic meaning of 'to be located' or 'to happen to be'. If you said je m'étais vite trouvé, it would mean 'I had quickly found myself [in a situation or place]' rather than finding your pocketknife.

Why use the plus-que-parfait (avais trouvé) here instead of the passé composé (ai trouvé)?

The plus-que-parfait (j'avais trouvé, meaning 'I had found') sets the background by describing an action completed before another past event. Even though the second event isn't in this sentence, it implies a wider story context, such as: 'I had quickly found my pocketknife [before we left the house].'

Is canif just another word for a knife?

The general word for a knife is couteau. You use canif specifically to talk about a small, folding pocketknife or penknife.