Learn Hardcore French: Quand il était malade, le médecin venait chez lui. - When he was sick, the doctor used to come to his house.

Était is the imparfait, which in French describes background situations or habitual actions in the past. Here it sets the scene—whenever he was sick—rather than pointing to a single, completed event.

Why is venait also in the imparfait rather than the passé composé?

Because venait describes a repeated or habitual action (“the doctor would come”) whenever the condition (his sickness) recurred. The imparfait is the tense of choice for such ongoing or customary past actions.

Is this sentence talking about one single visit or about visits that happened repeatedly?

It implies repeated visits. The imparfait in both clauses signals that every time he fell sick, the doctor used to come.

How would the meaning change if I used the passé composé: Quand il a été malade, le médecin est venu chez lui?

That version shifts to one specific past event: “When he got sick, the doctor came [on that occasion].” It no longer conveys habitual action.

Why is chez lui used, and what does chez mean?

Chez means “at the home/place of.” So chez lui literally means “at his place” or “at his home,” indicating where the doctor came.

Why is lui used instead of il after chez?

After prepositions like chez, French requires the stressed/disjunctive pronouns (moi, toi, lui, elle, etc.), not the subject pronouns (je, tu, il, elle).

Why isn’t there an article before malade?

With être, adjectives denoting a state or condition (like malade) are used without an article: you say être malade, not être un malade.

Can I change the word order? For example, put quand il était malade at the end of the sentence?

Yes. Le médecin venait chez lui quand il était malade is perfectly acceptable. Placing the time clause first (as in the original) simply emphasizes the background condition.