Difference between 加える and 足す?

Question

Both seems to mean "to add". What are the differences? Any other words that could be used alongside these?

Answer 1

For one, 加える is more formal than 足す。 足す can also mean to add, as in adding numbers / mathematics. The really small difference is that 足す　is generally used when adding to an already existing quantity, while 加える is more used when adding something new. Examples:

1. コーヒーに砂糖を加える。
2. コーヒーに砂糖を足す。

Both mean to add sugar to coffee, but the former would be used in a scenario when I am creating a latte or some other sweet coffee beverage. The latter means to add sugar, as in the coffee already has sugar, but it's still too bitter.