Some Nouns in english are countable and uncountable.
"Part" is countable and uncountable , what is the difference in meaning in each case?
The knee is apart of the leg
This piece of glass seems to be (a) part of a lamp
There is not a fixed rule you can learn here. You have to find a good dictionary and check the different meanings of the same word. Also, it's good to read example sentences. Sometimes countable and the uncountable words can have very similar meanings (e.g. work, experience) and sometimes not (e.g. iron).
You'll need to give us examples.