即使 cannot separate it. It must use it together. It can be used under certain conditions. Expressing... "no matter how", "even though", "even if" 1) 即使明天下雨，我也要出去玩. 2) 即使困难再大，我们也要想办法解决，决不能向困难低头 3) 即使今天下雨,我也要坚持上学.

There is another word called "就算“ which has exactly the same meaning as "即使".-- And we tend to use "就算" in oral communications and use "即使" in writing articles.-- As you said the two characters can be separated.-- "就" or "即" is the so called "function word" in Chinese.-- In modern Chinese "就" could be used to emphasize the emotion-- For example.-- 是我做的: I m the person who did it.-- 就是我做的: I m definitely the person who did it.-- 算 means "admit something" here, which is similar to "if".-- so "就算" means "even if".-- And so do "即使" -- Here is an example that may help you understand the word. ”算你厉害“----”(I admit that) you are strong“ ”就算你厉害，但你能打败他吗？“----”even if you are strong, can you defeat him?“.

使 here is like let/make，such as 使得，使人… e.g 这天气使得我不得不多加件衣服，实在是太冷了。 e.g 这回信使我/她/他/他们泪流满面。 Also 使 can mean to use sth. E.g 这东西不太好使。 e.g 这刀很锋利，记得使用的时候小心点哦。 in the term of 即使，apparently 使 means the former. Literally 即使的意思就是 let something happen 也就是even if (. =after sth has happened...)