Learn Croatian: Voda iz jezera je danas hladnija. - The water from the lake is colder today.

The base adjective is hladan (cold). First, we drop the 'fleeting a' from the masculine form to get the root hladn-. Then, to make it comparative, we add the regular suffix -iji (masculine), -ija (feminine), or -ije (neuter). Since voda is feminine, we add -ija to get hladnija (colder).

Why does the word for lake change to jezera here?

The sentence uses the preposition iz (from / out of), which always requires the following noun to be in the genitive case. The base word is the neuter noun jezero, and its genitive ending is -a, making it jezera.

Why does the little verb je come after the whole phrase 'Voda iz jezera'?

The verb je is a clitic, meaning it usually sits in the "second position" of the sentence. In Croatian, this second position can be after the very first word, but it can also be after the first complete thought or phrase. Here, Voda iz jezera acts as one single logical block, so je slides in right after it.

Why do we use iz for "from" the lake, rather than od?

The preposition iz specifically means "out of" the inside of a space, which makes sense for water taken out of a lake. If you used od (which generally means "from" a person or "away from" an outside surface), it would sound more like the water came from the general vicinity of the lake, rather than from inside it.