Learn Finnish: Iloiset lapset menevät kouluun. - The happy children are going to school.

In Finnish, adjectives must match the noun they describe in both case and number. Since lapset (children) is plural, the adjective iloinen (happy) must also take the plural marker -t, becoming iloiset.

How does the dictionary word iloinen turn into iloiset?

Words ending in -nen follow a special rule. When you add an ending like the plural -t, the -nen part changes to -se-. So, iloinen drops the -nen, adds -se-, and finally adds the plural -t to make iloiset.

Why is the verb menevät instead of menee?

The verb always matches the subject. Because the subject iloiset lapset is plural (they), we use the plural form of the verb mennä (to go), which is menevät.

Why does koulu have -un at the end?

The ending -un translates to into or to. When you are going to a place in Finnish, you use this form. You usually build it by doubling the last vowel of the word and adding -n, making koulu (school) into kouluun (to school).