Learn Dutch: Is de taart lekker?

To make 'little cake', you add the basic diminutive suffix -je to the end of the word to get taartje. Keep in mind that all diminutive nouns are het words, so de taart becomes het taartje.

Why does the sentence start with the verb Is?

In Dutch, just like in English, you form a yes/no question by moving the verb to the very beginning of the sentence. This swaps the normal order of the subject (de taart) and the verb (is).

Can I use lekker to describe things other than food?

Yes! While it means 'tasty' or 'delicious' when talking about food like de taart, Dutch people use lekker for almost anything pleasant. You can use it to describe nice weather, a comfortable chair, or sleeping well.

How would I ask this question in the plural (Are the cakes tasty)?

You would change the verb to the plural form zijn (are) and add -en to the noun to make it taarten. The adjective lekker does not change here. The sentence becomes: Zijn de taarten lekker?