Learn French: Le dormeur a souvent ronflé cette nuit. - The sleeper often snored last night.

In French compound tenses like the passé composé, short or common adverbs like souvent (often), bien, or déjà are placed directly between the auxiliary verb (a) and the past participle (ronflé). English is more flexible with adverb placement, but French strictly requires tucking these short adverbs inside the two-part verb structure.

Doesn't cette nuit mean 'tonight'? Why is it translated as 'last night'?

Cette nuit literally means 'this night', but its meaning shifts based on the tense of the sentence. Because this sentence is in the past tense (a ronflé), it refers to the night that just passed ('last night'). If the sentence used a future tense, it would mean 'tonight'.

Since he snored 'often', why is the verb in the passé composé instead of the imparfait?

Even though souvent (often) suggests a repeated action, the timeframe is limited to one specific, completed period (cette nuit). When an action happens within a clearly defined, closed window of time in the past, French uses the passé composé, no matter how many times the action was repeated during that window.

How is the noun dormeur formed?

It is built from the root of the verb dormir (to sleep). Just like English adds '-er' to a verb to describe someone who does that action (sleep becomes sleeper), French often adds the suffix -eur to a verb stem to create a similar noun.