In English, we say "One book" (Number first, noun second). But Arabic handles numbers `1` and `2` very differently: They are treated exactly like regular adjectives! First you state the noun, then you use the number to "describe" it!
Remember the Mirror Rule for adjectives? The number `1` acts exactly the same. It comes after the noun and perfectly mirrors its gender (adding a Ta' Marboota for females).
To count `2` of something, we first make the noun Dual (`-aani`), and then use the matching Dual adjective: `اِثْنَانِ` for males, and `اِثْنَتَانِ` for females!
كِتَابَانِ (Two books) and سَيَّارَتَانِ (Two cars) already mean "Two", the word "Two" (Ithnaani/Ithnataani) is actually optional! Arabs use it purely for EMPHASIS ("I have exactly two cars!").
Read aloud. Notice how numbers 1 and 2 flawlessly mirror the noun's gender, exactly like regular adjectives!