Can anybody explain to me the use of the indefinite article in the following sentences:
- She had a great zest for life.
- The danger of being caught added a certain zest to the affair.
"Zest" is a non-count noun, of course.
On the broader point, adjectives are partitive. Green apples are a particular type of apples in the category of all apples. Green apples are part of all apples.
The same thing applies with uncountable nouns.
A little/deep/good/wide/slight/simple, etc., knowledge.
Zest = the entire category of things that can be described as "zest".
Certain (adj.) = a particular type of
A/an = an example of [a/an]
So "a certain zest" is not all things that can be described as "zest" but only a an example of a part of the category of "zest".
There are many parts to the entirety of "zest" and a certain zest is one example of one of those categories.