A subject pronoun is exactly what it sounds like: a pronoun that takes the place of a noun as the subject of a sentence. Remember, a sentence's subject is the person or thing that performs the action of a verb. When you take an even closer look, you'll see that a subject pronoun is used as the subject of a verb, while an object pronoun is usually used as a grammatical object.
Subject pronouns can be singular or plural, and they can be masculine, feminine, or gender neutral. The masculine or feminine subject pronoun is used when gender is known; when referring to an inanimate object, the gender-neutral form "it" is used. The subject pronoun "it" can be used to refer to animals of unspecific gender, and it is also appropriate to use the subject pronoun "it" to describe a baby of unknown gender. "It" is also used to talk about the weather, temperature, or time.
If you can find the subject of a sentence, then you can find a subject pronoun just as easily! Let's go back to basics for a moment. Every complete sentence has a subject, a verb, and an object. The subject (or subject pronoun) is always a person, thing, place, or idea, or the plural of one of those, i.e. people, things, places, or ideas. The object or object pronoun is always on the receiving end of the action.
One of the easiest ways to identify a subject or subject pronoun is to remember that it is always going to be the who or what that has a direct effect on the action that's taking place.
In the following examples, you can see exactly how this method works. The subject pronoun is in bold and is underlined, the verb is in italics, and the object is in bold.