Adverb. Degrees of adverbs

References:

1. 
   Crystal, D. (1999). The Cambridge Encyclopedia of Language. Cambridge: Cambridge University Press.
   
2. 
   Huddleston, R., & Pullum, G. K. (2002). The Cambridge Grammar of the English Language. Cambridge: Cambridge University Press.
   
3. 
   Quirk, R., Greenbaum, S., Leech, G., & Svartvik, J. (1985). A Comprehensive Grammar of the English Language. London: Longman.
   
4. 
   Biber, D., Johansson, S., Leech, G., Conrad, S., & Finegan, E. (1999). Longman Grammar of Spoken and Written English. London: Pearson Education Limited.
   
5. 
   Hudson, R. (2010). An Introduction to Word Grammar. Cambridge: Cambridge University Press.