Learn Finnish: Sinun ei tarvitse valita sokeritonta kakkua. - You do not need to choose a sugar-free cake.

It comes from the noun sokeri (sugar) plus the suffix -ton meaning 'without' or '-less', creating the adjective sokeriton (sugar-free). Here, it has the partitive ending -ta so it matches the noun it describes, kakkua.

Why are sokeritonta and kakkua in the partitive case?

Because the sentence is negative (ei tarvitse). In Finnish, the direct object of a negative sentence is almost always in the partitive case, so both the noun and its attached adjective must take the partitive form.

Why does the sentence start with the genitive sinun instead of the normal 'you' (sinä)?

This is because of the 'necessive' structure, which expresses necessity or obligation. When you use a verb like tarvita (to need) or täytyä (must), the person who needs to do the action is put into the genitive case.

Why doesn't the verb tarvitse have the 'you' ending (-t) since the subject is 'you'?

In necessive sentences, the verb never takes personal endings matching the subject. Instead, it acts a bit like a fixed phrase. Since this sentence is negative (ei), we use the basic negative stem tarvitse for all subjects (I, you, he, we, etc.).