Use はもとより to say that something applies to a primary thing and also to something else that is less expected or less obvious. It is often translated as 'not to mention' or 'let alone'. For example, if you say 'He is good at math, not to mention English', you use はもとより for the primary subject. It emphasizes that the second point is even more true or surprising. This is a formal expression, common in written Japanese and formal speeches, not in casual conversation. It can also mean 'from the beginning' when attached to a time expression like 最初はもとより, but that usage is less common and more literary. In practice, you'll mostly see it as 'not to mention'.