Based on whether they can be counted and used with numbers or not, nouns can be divided into two categories. Countable nouns (sustantivos contables) are those that can be used with numbers and be counted, while uncountable nouns (sustantivos incontables) refer to thing that cannot be counted and appear as a mass.

These nouns refer to discrete things that can be counted, such as people, objects, animals, etc.
They can appear in singular or plural, and can be used with numbers and indefinite articles (un/una).

Here are some examples:

These nouns refer to substances, materials, or abstract concepts that can't be divided into individual units without using a measure word.
They generally do not have a plural form, and do not take numbers or indefinite articles (un/una). In order to refer to the amount of these nouns, indefinite quantifiers like mucho, poco, bastante, etc. are used. Here are some of the common uncountable nouns in Spanish:

Here are some example sentences:

Since uncountable nouns cannot be directly used with a number, we use units of measurement or containers to quantify them. For example:

As already discussed, uncountable nouns are not used in plural forms because they refer to a mass or substance that cannot be counted. However, sometimes, these nouns might be used in plural form. In these cases, the plural shows that the speaker is talking about different types or multiple units of the uncountable substance. Compare the examples:

When countable nouns follow verbs as the direct or indirect object, they need an article, quantifier, or number to modify them. Uncountable nouns, however, can appear without any article or quantifying modifier.