Learn Romanian: Cartea costă treizeci de lei.

In Romanian, when you count things using numbers from 20 onwards, you must put de between the number and the noun. It acts a bit like saying "thirty of lei". For numbers 1 to 19, you just put the number directly in front of the noun without de.

How is the word treizeci formed?

It is made by combining trei (three) and zeci (tens, the plural of zece). Most Romanian numbers in the tens follow this pattern, literally saying "two-tens" (douăzeci), "three-tens" (treizeci), and so on.

Why is the word for "the book" cartea instead of just carte?

Carte means "book". To say "the book", Romanian attaches the word for "the" to the very end of the noun. For feminine words ending in -e like carte, the ending changes to -ea, making it cartea.

What is the singular form of lei?

The singular is leu. It is the name of the Romanian currency, but it also literally translates to "lion". To make it plural, the ending changes from -u to -i, making it lei.