These are generally attributive only - ie they pre-modify nouns - eg an utter idiot; mere folly; sheer foolishness; total rubbish. They cannot be predicative - ie occurring as complement : *The idiot was utter; * His foolishness seemed sheer; *The rubbish looked total etc.
These may pre-modify either verbs - I really liked it, or adjectives - It was incredibly hot in there.
Quirk et al (ibid) divide them into three categories and further subcategories :
a) Emphasizers - eg definitely, certainly, clearly, obviously, really etc: It's obviously wrong.
b) Amplifiers, which subdivide into
- Maximisers - absolutely, completely, thoroughly, totally etc: I'm totally convinced that....
- Boosters - deeply, greatly, terribly, highly, very much etc: He deeply regretted saying that....
c) Downtoners, which have four subcategories :
- Compromisers - kind of, sort of, rather etc - I was kind of expecting it.
- Diminishers - partially, mildly, somewhat etc : He's somewhat strict but....
- Minimizers - a bit, hardly, scarcely, in the slightest etc : I could hardly see through the smoke.
- Approximators - almost, nearly, virtually, practically etc : It's practically impossible.
Intensifiers may be repeated to give extra emphasis : He was really, really tired.
Quirk, R., Greenbaum, S., Leech, G., and Svartvik, J., 1972, A Grammar of Contemporary English. London: Longman