English question about singular countable nouns and countable nouns?

I am quite confused about the usage of countable nouns and singular countable nouns. I would like to know about their usages.
I will try to explain those with some examples.
1. Knowledge: In dictionaries the type of noun is given as uncountable and singular.

It can be singular in following instances:A strong knowledge of computer or a knowledge of carpentry. Actually, we can’t measure knowledge – 1 knowledge, 2 knowledge, etc.

Let’s take another example: life or work

Life: Per dictionary, it’s an uncountable noun as well as a countable noun. But, I have heard lots of people saying, you are living a good life. If you set this example against the first one, then you can see the difference. Life is countable, and we can use an indefinite article and knowledge is a singular noun and we can use an indefinite article.

1. You are living a good life. After contemplating it more, I am now assuming we can use an indefinite article if it is a countable or singular countable noun. But, if we consider work, it is more used as an uncountable noun. Sometimes it is used as a countable noun too. If it is used as a countable noun, then why can’t we say –

1. You are doing a good work.

However, we can say “a work in progress”.

Similarly, with “fruit” and “food” -Give me a fruit or give me fruit or give me some fruits.Can someone elucidate the difference to me? Or Should I have to check their usages before using them in sentences?

You can check out the types of nouns in Merriam Webster’s advanced learner’s dictionary.