Learn Finnish: Kyllä sinä muistat vaihtaa kaistaa ajoissa. - You will certainly remember to change lanes in time.

Here, kyllä is used as an epistemic adverb to assert that the statement is absolutely true. It adds a tone of reassurance or strong confidence, which is why it translates to certainly or surely in English.

Why is the object kaistaa in the partitive case instead of the accusative?

The verb vaihtaa (to change) often takes the partitive when the action is considered a continuous process rather than a singular, finalized achievement. In fact, vaihtaa kaistaa is a standard set phrase for changing lanes, and it always takes the partitive.

Ajoissa looks like the plural inessive form of aika (time). Is that how you say in time?

Exactly. While it literally translates to in times, ajoissa is a fixed adverb meaning in time or early enough. It is very common to see time-related adverbs use plural local cases in Finnish.

Are there any special rules for combining muistaa with another verb like vaihtaa here?

When muistaa means to remember to do something, it requires the next verb to be in the first infinitive (the basic dictionary form). Unlike some other verbs that require the third infinitive (like oppia or auttaa), you don't need any special endings to link them, so muistat vaihtaa connects directly.