Have a look at this sentence:
I have noticed that paragraphs 81 and 82 are virtually similar to paragraphs 5 and 6.
“Virtually similar” is incorrect because “virtually” is a non-grading adverb and “similar” is a gradable adjective.
“Very similar” or “virtually identical” are correct.
Think about what “similar” means. Things that are similar to each other have attributes in common. Some similar things may have almost all of their attributes in common; some similar things may have only a few attributes in common. So there are degrees of similarity. “Similar” is a gradable adjective.
“Identical”, on the other hand, is a non-gradable adjective. Something is identical to something else, or it is not.
“Virtually” is a non-grading adverb and can only be used with non-gradable adjectives. Saying “virtually similar” is the same as saying “almost similar” – which is absurd. Instead you should say, for example, “very similar”, “quite similar” or “rather similar”. “Very”, “quite” and “rather” are grading adverbs.
| Examples of grading adverbs | Examples of gradable adjectives |
| extremely, hugely, immensely, intensely, quite, rather, reasonably, slightly, tremendously, very | big, busy, common, deep, fast, high, important, popular, rich, strong, young |
| Examples of non-grading adverbs | Examples of non-gradable adjectives |
| almost, absolutely, completely, entirely, exclusively, fully, mainly, nearly, perfectly, primarily, simply, totally, utterly, virtually | awful, essential, excellent, huge, identical, impossible, perfect, terrible, unique, unknown, whole, wonderful
|
Adjectives that classify a thing, such as annual, digital, domestic, environmental, general, maximum, minimum, northern, underlying, etc. are also non-gradable. A thing is either annual, digital, domestic etc., or it is not.
The adverbs fairly, really and pretty can generally be used with both gradable and non-gradable adjectives.
Some adjectives can be both gradable and non-gradable, depending on their meaning.
| Gradable meaning | Non-gradable meaning (including classifying adjectives) |
| It is very common for employers to offer overtime pay.
My old boss was very critical of my work. My computer is very old. My cat is very particular about his food. The new graphic designer has some very original ideas. They are very private people. We seldom see them out of their house. They had a very public argument on the train – everyone could hear. |
We have a lot of common interests.
This hearing is absolutely critical for the case. My old boss was very critical of my work. How we deal with this issue depends on the particular activity. Please send me the original version of the document. It has been a private company ever since it was established 100 years ago. It became a public company in 2015. |
Another important question to consider when pairing adverbs with adjectives is – do they collocate? Collocations are combinations of words that commonly occur together. So, although it may be grammatically correct to say e.g. “hugely deep” (as we’re pairing a grading adverb with a gradable adjective), native speakers do not usually use this word combination. Instead we might say “extremely deep” or “tremendously deep”.
There are innumerable collocations, and it would be impossible to learn them all, except through continued exposure to native-speaker language.
A final point to make is that there are numerous exceptions to these rules (as always!). If you are in doubt about a particular adverb-adjective pairing, try a Google search. If the word pair gets a significant number of hits, it is probably OK; if not – try another pairing. For example, “hugely deep” only gets 11,100 hits, while “extremely deep” gets over a million.
Common Mistakes in Business English 
I would like a paper on collocation of adverbs and adjectives if you know of one of it; I would appreciate you let me know.
Hi Casimiro – I suggest you google “collocation of adverbs and adjectives”. I just tried it and loads of pages came up. You could also read as much native-English writing you can find and make a note of the collocations.