forty = 40

forty = 40
four, fourteen, but no 'u' in forty

Add a hyphen in forty-one, forty-two, forty-four, etc.

Change the 'y' to 'i' forty — fortieth (40th)
Change the 'y' to 'ies' in forty — forties (40s)
But fortyish (or else 2 i’s together)