以 is a classical Chinese function word that remains very productive: 可以 (can), 以后 (after/later), 以前 (before/ago), 以为 (to think/assume), 以上 (above), 以下 (below), 所以 (therefore).
grammar
以 has a formal/literary tone. In patterns like 以...为... (to regard...as...) or 以便 (so that), it adds elegance to writing.
