Nouns are the most common type of subjects. They include people, places, and things and can be concrete (book, chair, house) or abstract (belief, notion, theory).
Example: Bats are able to hang upside down for long periods because they possess specialized tendons in their feet.
Pronouns are words that replace nouns. Common pronouns include she, he, it, one, you, this, that, and there.
Less common pronouns include what, how, whether, and that, all of which are singular. They are typically used as part of a much longer complete subject (underlined in the second example below).
Example: They are able to hang upside down for long periods because they possess specialized tendons in their feet.
Example: How bats hang upside down for long periods was a mystery until it was discovered that they possess specialized tendons in their feet.
Gerunds are formed by adding -ING to the ends of verbs (e.g. read – reading; talk – talking). Although gerunds look like verbs, they act like nouns. They are always singular and take singular verbs.
Example: Hanging upside down for long periods is a skill that both bats and sloths possess.
The infinitive is the “to” form of a verb. Infinitives are always singular when they are used as subjects. They are most commonly used to create the parallel structure “To do x is to do y.”
Example: To hang upside down for a long period of time is to experience the world as a bat or sloth does.