Learn Hardcore Japanese: titi ga unten dekinai hi ha, haha ga kuruma wo untensimasu. - On days when my father can’t drive, my mother drives the car.

In the subordinate clause 運転できない, 父が marks “father” as the subject—the one who cannot drive. In Japanese potential forms (～できる/できない), the subject is typically marked with が to emphasize ability or inability.

Why is there no particle between 運転できない and 日?

Japanese uses relative clauses without extra connectors. You directly stack the clause 運転できない (“cannot drive”) in front of 日 (“day”) to mean “the day when [someone] cannot drive.” There’s no need for a word like “when” or a relative pronoun.

What does 運転できない mean, and how is it formed?

運転する = “to drive.”
To express ability, you use the potential form of する, which is できる.
Thus 運転できる = “can drive,” and 運転できない = “cannot drive.”

Why is は used after 日?

The particle は marks 日 (“day”) as the topic of the main clause. It tells the listener: “As for the days when father can’t drive, …” This shifts focus to those days as the setting for what follows.

Why are が and を used in the second clause 母が車を運転します?

母が: が marks “mother” as the subject doing the driving.
車を: を marks “car” as the direct object being driven.
運転します is the polite present form of “to drive.”

Why is the main clause verb in polite form (運転します) but the subordinate clause in plain form (運転できない)?

It’s common in Japanese to mix forms: subordinate or embedded clauses often use the plain form, while the main clause uses polite speech. This keeps the overall statement polite without forcing every element into ～ます/～です form.

Can you replace 日 with 時 to say “when father can’t drive” instead of “the day”?

Yes, but the nuance shifts:

日 emphasizes the entire day or those days.
時 (toki) focuses on a specific moment or period.
So 父が運転できない時は… means “When father can’t drive (at that time)…,” whereas 日 implies “On days when father can’t drive….”