What is the simple subject in this sentence: The Andes Mountains were home to the Incas.

The correct answer is: "Andes Mountains".

The simple subject in the sentence: The Andes Mountains were home to the Incas is: "Andes Mountains".

It is important to refresh our memories with the following facts:

*The subject of a sentence is one of the two main parts of a sentence (the second one is the predicate); it usually appears before the predicate to express who or what is being talked about, that is, a noun (person, thing, etc.) or pronoun.

*Sometimes the noun will have some modifiers (such as adjectives), in such cases the subject will be composed of the noun plus all the words which modify the noun, that will be the full or complete subject.

*The simple subject, in contrast, is only one word, that is, the noun or pronoun left after we remove all the modifiers.

In the sentence: The little white cat run across the street, the subject or full subject is: "The little white cat". This sentence is talking about a cat, but not any cat, but rather "The little white cat".

But, if we want to underline only the simple subject then it shall be just the word "cat" removing all the modifiers.

Now, why is it then that in the sentence of our question the answer is "Andes Mountains"? That is because it is a proper noun. It is the name by which that famous mountain range in South America is known by. Proper nouns may contain two or more words, but are still referring to a single concept. They become a compound noun.

Just as this last rule regarding the subject in a sentence there are several more to take into account when we want to find the full or simple subject in a sentence.