Learn Croatian: Vjetar je slabiji u dolini. - The wind is weaker in the valley.

The base adjective for weak is slab. To make it comparative (weaker), we add the regular suffix -iji. Because vjetar (wind) is a masculine noun, the adjective takes the masculine nominative form slabiji.

If the noun was feminine, like oluja (storm), how would the comparative change?

Adjectives in the comparative form still must agree with the noun's gender. If you were talking about a storm, you would add the feminine ending -a to the comparative base, making it Oluja je slabija.

Why is the word for valley spelled dolini here instead of dolina?

We are expressing a static location using the preposition u (in), which triggers the locative case. For feminine nouns that end in -a like dolina, the locative case ending is -i, resulting in u dolini.

Why is the verb je placed exactly where it is?

The verb je (is) is an enclitic, meaning it is a short, unstressed word that cannot start a sentence. It naturally falls into the second position in the sentence, immediately following the first word or phrase.