In one of my all-time favorite movies, The Princess Bride, there is a scene where the main character Wesley, after having been tortured by the evil prince’s henchman, is brought to Miracle Max in order to be restored. Wesley’s companion Inigo thinks that Wesley is dead, but Miracle Max, the wise old man he is, corrects Inigo, saying, “It just so happens that your friend here is only mostly dead.”
Miracle Max, The Princess Bride
Part of the scene’s humor, of course, is that most people are not aware of a state of being where one is “mostly dead.” Either a human is dead or not; there is no in-between space. While Miracle Max may have special insight that allows him to make this distinction, his statement brings up the grammatical issue of gradable versus non-gradable adjectives.
According to traditional grammar, all adjectives in the English language can be placed into one of these two categories. A gradable adjective can vary in degree or intensity and can therefore have a superlative form. A non-gradable adjective, meanwhile, does not vary in degree or intensity and thus has no accompanying superlative form. Let’s look at some examples.
GRADABLE ADJECTIVES
Large
“Large” is a gradable adjective because there are varying degrees of largeness. 
A building might be large, but the building next to it could be larger, and the skyscraper down the street may very well be the largest of them all. You can also say things like “very large” and “extremely large,” again suggesting a spectrum of different intensities.
Fun
Even when you can’t add “-er” or “-est” to a word, it may still be a gradable adjective. Such is the case with the word “fun” (and many, many other adjectives). It would be acceptable to say, “Playing soccer is more fun than watching soccer.” In this example, both playing soccer and watching soccer are fun, but one is more fun than the other. And just like we can conceive of a building being “the largest,” so, too, we might say that a certain activity is “the most fun.”
NON-GRADABLE ADJECTIVES
Impossible
Unlike “large” and “fun,” a word like “impossible” cannot have various shades of intensity. Either a task is possible, or it is impossible. You cannot compare two impossible things, for if one were less impossible than the other, then it would necessarily be possible. Likewise, it is incorrect (“incorrect” being another non-gradable adjective, by the way) to say that something is “very impossible,” “most impossible,” “extremely impossible,” etc.
Unique
In conversational English (and sometimes written English, too), many speakers mistake “unique” for a gradable adjective, saying things like “that painting is so unique” or “she has a very unique personality.” The word “unique” is by definition non-gradable, since it literally means one of a kind, completely unlike anything else. There is no sense, then, in saying that something is “more unique” than something else, or that a person is the “most unique” you’ve ever met. Either a thing is unique or it’s not; the word is non-gradable. This happens to be a very common error, so beware of it in your own writing.
EXCEPTIONS
Although non-gradable adjectives are said to be non-modifiable, they can be accompanied by certain adverbs in order to intensify or dilute them. Some of these adverbs are:
completely, totally, absolutely, utterly [to intensify]
partly, nearly, almost [to dilute]
We’ve already covered the fact that a task is either possible or impossible and cannot fall anywhere between. Nonetheless, it is still acceptable to call something, say, “absolutely impossible,” as that provides emphasis. You can also use adverbs to de-emphasize non-gradable adjectives, as in the sentence, “Swimming the entire length of the English Channel without any support is a nearly impossible undertaking.” In this sentence, it is understood that the task is indeed possible, but it is very, very difficult – bordering, in fact, on being impossible.
It is somewhat of a mystery that adverbs like “absolutely” and “nearly” are allowed to pair up with non-gradable adjectives while other adverbs such as “very” are not. And to confuse matters even further, there are some instances – in conversational English, anyway – where even that rule gets broken.
EXCEPTIONS TO THE EXCEPTIONS
In some situations, a speaker might choose to add emphasis to a non-gradable adjective by using what appears to be an inappropriate adverb. For instance, the word “pregnant” is generally regarded as being non-gradable; either a woman is pregnant or not. However, it is not uncommon to hear people say something like “she is very pregnant” or “my sister is feeling really pregnant nowadays.” Such expressions suggest that a woman is further along in her pregnancy (e.g. in her third trimester rather than her first), which, in a sense, could be considered “more” pregnant. When thus viewing states of being in terms of processes, it becomes clear what people mean by using such adverbs of degree with non-gradable adjectives. If we consider death in terms of a process rather than a state of being, then we see what Miracle Max means when he calls Wesley “mostly dead,” and we wouldn’t be too off-put if he later referred to his patient as “very dead” or “somewhat dead,” either.
All told, saying that we can classify all adjectives into one of two categories might assume more rigidity in the English language than there actually is. As a general rule, try to avoid using non-gradable adjectives in a comparative or superlative way, especially in formal writing situations. But, if the context calls for it, you can, à la Princess Bride, formulate your sentences “as you wish.”
