As I understand, the word "however" is synonymous to "but", except that it can only be used, to grossly simplify, at the beginning of sentences, like this:
"It was red. However, it was hard to see."
I am also coming to believe that it may be inserted in the middle of a phrase, if that phrase is near the beginning:
"It was red. It was, however, not easy to see."
Finally, I believe it is incorrect to use it as a substitute of "but", like this:
"It was red, however it was hard to see."
Is any of what I am saying here correct, if not, what are?