First of all, here are the personal subject pronouns in Spanish:
- yo — I
- tú — you
- usted — you (singular formal)
- él, ella — he, she
- nosotros, nosotras — we
- vosotros, vosotras — you (plural familiar)
- ustedes — you (plural formal)
- ellos, ellas — they
Because the verb form often indicates who the subject of a sentence is, one can properly leave out the subject pronoun or put it at various places in the sentence. Voy a la escuela, yo voy a la escuela, voy yo a la escuela, and voy a la escuela yo are all grammatically correct ways of saying "I am going to the school" (although the final option would be uncommon except if said for poetic effect). But the placement of the pronoun can make a difference in how the sentence is understood.
Example:
- Mi hermano es muy inteligente. Es doctor. (My brother is intelligent. He's a doctor.) — No subject pronoun is needed in the second sentence, because the subject of the sentence is made clear by the context and the verb form. Generally, and especially in writing, unneeded pronouns aren't included unless there's a reason for doing so.
- Mi hermano y su esposa son inteligentes. Él es doctor, y ella es abogado. (My brother and his wife are intelligent. He is a doctor, and she is a lawyer.) — In this case, the subject pronouns él and ella are needed for clarity.
- Hazlo. (Do it.) Hazlo tú. (You do it.) — In a command such as this, the addition of the subject often has a similar effect to its use in English. Although grammatically not necessary, the addition of the subject serves to place additional emphasis on the subject.