Hebrew numbers are so hard and crazy that my Hebrew teacher refuses to cover them in first year Hebrew (he’d rather wait and discuss them as they appear in texts)!!
The forms of the numbers are simple enough – they just need to be learnt like all other words and vocabulary. But it is how the numbers work that is strange:
Number ‘one’ functions like an attributive adjective (meaning that it agrees with its noun in number, gender and definiteness).Number ‘two’ and all other cardinal numbers function more like substantives.
Cardinal numbers 3-9 and 13-19 disagree with their noun in gender. So a masculine noun is used with a feminine number; and a feminine noun is used with a masculine number.
A similar thing happens with the numbers 100 (feminine) and 1000 (masculine). To make the number 400, you would combine the masculine form of ‘four’ with the feminine form 1000. To make 4000, use the feminine 4 and masculine 1000.
Just to ease the pain slightly: ordinal numbers (“first”, “second” etc.) are pretty regular and don’t have any kind of confusion or gender reassignment operations!