Fairly is an adverb of degree. It generally modifies adjectives and adverbs.

• He can speak English fairly well.

Fairly does not suggest a very high degree.

• ‘How was the film?’ ‘Fairly good.’ (Not the best one I have seen this year.)

Quite

Quite suggests a higher degree than fairly.

• ‘How was the film?’ ‘Quite good.’ (You ought to watch it.)

Quite can modify adjectives, adverbs, verbs and nouns.

• She speaks English quite well.
• He is quite tall.
• He is quite a scholar.
• I quite enjoyed myself at the party.

Rather

Rather is stronger than quite. It suggests ideas such as ‘more than is usual’, ‘more than was expected’ or ‘more than was wanted’.

• I think I should close the window. It is rather cold.
• Do you see that rather tall boy standing over there.
• ‘How was the film?’ ‘Rather good.’ (I was surprised.)

Rather can modify adjectives, adverbs, verbs and nouns.

• It was rather a success.
• I rather think we are going to lose.

Pretty

Pretty is similar to rather.

• She is a pretty good girl.
• ‘How is things?’ ‘Pretty good.’

Pretty can modify adjectives and adverbs. It can’t modify nouns or verbs.