Learn Croatian: Grupa studenata uči hrvatski. - A group of students is studying Croatian.

To say "of students", we use the genitive plural. The regular ending for this is -a. However, since the genitive singular (of one student) is already studenta, the plural needs to look and sound different. To fix this and to break up the hard "nt" consonant combination at the end of the root word, Croatian inserts an extra -a- before the final consonant. This makes it studenata.

If there are many students, why is the verb uči (he/she/it studies) instead of uče (they study)?

The verb always matches the main grammatical subject of the sentence. Here, the subject is grupa (a group), which is a singular feminine noun. Because there is only one group, we must use the singular verb uči. The word studenata just describes the group, so it doesn't control the verb.

There is no word for "of" in this sentence. How do we know it means "a group of students"?

In Croatian, the genitive case itself does the job of the English word "of". By putting the noun into the genitive form (studenata), the meaning of "belonging to" or "made up of" is built right into the word. You don't need to add a separate preposition.

Why is the word for the Croatian language just hrvatski here?