Learn Croatian: Tko je peti klijent? - Who is the fifth client?

In Croatian, ordinal numbers (first, second, third, etc.) act like adjectives. You usually take the regular number like pet and add adjective endings. Because klijent is masculine, we add the masculine ending -i to make peti.

Why is the ending -i used on peti instead of -a or -o?

Ordinal numbers must match the gender of the noun they describe. The word klijent is masculine, so we use the masculine singular nominative ending -i. If we were talking about a feminine noun like osoba (person), it would be peta osoba.

Does every number turn into an ordinal word just by adding an ending, exactly like pet to peti?

Not always! While pet perfectly becomes peti, some numbers change their spelling slightly. For example, three (tri) becomes treći and four (četiri) becomes četvrti. Also, first (prvi) and second (drugi) are completely different words from one and two.

Could I say Koji je peti klijent? instead of using Tko?