I believe I have found the answer to this after going through several other lessons and some practice.
Looks like it was an easy one. But Spanish experts please correct me if I am wrong.
Seems like it's quite similar to English.
If a noun is something quantifiable, then we use the article in front of the noun.
Example: apples are quantifiable.
In English: It is similar to having 'a' or 'an' in front of a noun. We would say an apple.
In Spanish: We use the appropriate article in front of the noun (based on gender and plurality). We would say una manzana.
If a noun is something unquantifiable, then we do not use the article in front of the noun.
Example: milk is not quantifiable.
In English: It is similar to using the noun without a or an in front of it. We would say milk.
In Spanish: An article will not be used before the noun. We would say leche.
I believe the exceptions would be:
- With countries as mentioned in the link shared by @Flimzy
- With abstractions as mentioned by @Walter Mitty
Let me know if there are other exceptions to this.