The plane flew. To find the subject you need to ask which noun in the sentence is doing the action. Find the verb first. The verb is flew. What flew? The plane flew.

Now add a prepositional phrase to the sentence.

The plane flew above the land. What flew? The plane flew. The prepositional phrase gives you details about the subject. Where did the plane fly? The plane flew above the land. Land is a noun, but it is not the subject of the sentence because the land did not fly rather the plane did. Land is the object of the preposition.

There does not have to be a preposition in a sentence. A sentence consists of three ingredients: a subject, a verb and a complete thought. Prepositions give details about the subject.

Example:

Alice sings. This is a complete sentence because there is a subject, verb and a complete thought.

Let’s add a prepositional phrase.

Alice sings after school. After is the preposition. School is the object of the preposition. The prepositional phrase tells when Alice, the subject, sings.