Learn Croatian: Moj prijatelj treba polovicu gline za lončarsko kolo. - My friend needs half of the clay for the potter's wheel.

Both mean "half", but polovica is a standard feminine noun, so it changes to the accusative case polovicu because it is the direct object of the verb treba. The word pola is an adverb and never changes its ending. You could also say treba pola gline, but using the noun polovicu is a great way to practice how fractions decline!

Why is the word for clay (glina) in the genitive case (gline) rather than the accusative?

Whenever you use a fraction noun like polovica, trećina (a third), or četvrtina (a quarter), the substance being divided must be in the genitive case. You can think of it as saying "a half OF the clay". Since glina is a feminine noun ending in -a, it takes the -e ending in the genitive singular.

I remember learning that trebati (to need) is often impersonal, like meni treba. Why is the subject moj prijatelj in the nominative here?

When trebati is followed by a noun as a direct object (like polovicu), it can be used as a regular verb where the person who needs something is the nominative subject. The impersonal structure (mojem prijatelju treba...) is strictly required when the "need" involves another verb, but with physical objects, both structures are correct and widely used.

Why does (potter's wheel) keep its basic dictionary form after the preposition ?