These words are adverbs, meaning that they describe verbs (action words), adjectives (words that describe nouns), and other adverbs. I found that with these words, I just use them instinctively, but if you need to find the difference between very similar words, there is always the dictionary! Cambridge Dictionary Online has very good, straightforward definitions. According to Cambridge, "Never" means "not at any time or occasion." "Ever" means "at any time." "Forever" means "for all time." Examples: "I will never trust him again." "Have you ever seen a coyote?" "We will be linked together by blood forever." ("Blood" here refers to the bloodline of a family.) The spoken phrase "never, ever" emphasizes the "not at any time," and "forever and ever" means "for all time, at any time." Examples: "I will never, ever see him again." "I will stay here forever and ever." Hope this helps! =)