What parts of speech can be subjects

Nouns 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 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 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.    Infinitives   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.