That may be true for functions which operate on a generic Monad typeclass, but typeclasses (Higher kinded types) aren't required to implement concrete Functor / Applicative / Monad instances for a particular type, e.g. Maybe:
fmap f Nothing = Nothing
fmap f (Just x) = Just (f x)
pure x = Just x
Nothing >>= f = Nothing
Just x >>= f = f x
When people say they don’t understand Monad, my first assumption is that they are wearing their ignorance as a badge of honor.
Otherwise memorize a little bit and dispose of the mystery. It’s not that far out.