So is the difference between "het" and "de" just gender? or is one more formal than the other?
The Dutch word 'het' also just means 'the' and 'it'; so you can say, "Het meisje eet een appel." (The girl eats an apple), or "Het is een meisje." (It is a girl). The Dutch word 'de' just means 'the' in some male and female nouns in the Dutch language. It's a very confusing decision to the use of 'de' and 'het'; because 'het' is for all of the neuter/neutral nouns like 'brood', 'water' or 'sap'.