Learn Dutch: De taartjes zijn niet zoet. - The little cakes are not sweet.

The base word is taart (cake or pie). First, we add the basic diminutive suffix -je to make it small, which gives us taartje (little cake). Then, to make it plural, we add an -s, because all plural diminutives in Dutch end in -s. That results in taartjes.

I thought all diminutives (words ending in -je) were "het" words. Why does this sentence use "de"?

You are right that singular diminutives are always het words, so "the little cake" is het taartje. However, all plural nouns in Dutch use the article de, no matter what their singular form is. Since taartjes is plural, it must be de taartjes.

Why is it just zoet and not zoete? Don't adjectives usually get an -e added to them?

Adjectives usually get an -e ending when they come directly before the noun (like de zoete taartjes). But in this sentence, zoet comes after the verb zijn (are) to describe the cakes. When an adjective is used later in the sentence like this, it stays in its basic form and never gets an extra -e.

Why is niet placed right before zoet?

In Dutch, when you want to negate a specific adjective to say something is not a certain way, you put the word niet (not) immediately before that adjective.