Lines and Basic Qualities
Before building complex shapes, it helps to know how to describe lines and curves. The word for a line is the feminine noun crta.
You can use adjectives like ravan (straight) and okrugao (round). Like all adjectives, they must agree with the gender of the noun they describe. For example, put (path) and stol (table) are masculine nouns, so they use masculine adjective forms.
Flat Geometric Shapes
Most basic two-dimensional shapes in Croatian are masculine nouns, including kvadrat (square), trokut (triangle), krug (circle), and pravokutnik (rectangle).
Because these are inanimate masculine nouns, their form remains exactly the same whether they are the subject of a sentence or the direct object.
Three-Dimensional Shapes
When discussing three-dimensional objects, genders vary. The word valjak (cylinder) is masculine, so it behaves like the 2D shapes above.
However, kugla (sphere) and piramida (pyramid) are feminine nouns ending in -a.
Like other feminine nouns, their endings change depending on their grammatical role. For example, after the preposition s (meaning "down from" or "off of"), the noun takes the genitive case, changing the -a ending to -e.