Yo vs. mí
The Spanish words yo and mí can both be translated as I or me in English. The key difference lies in their grammatical function: yo is used as the subject of a sentence, while mí is used after a preposition.
Yo
A1Yo is a subject pronoun. It is used when I am the person performing the action of the verb. It is the direct equivalent of I in English when I is the subject. While often omitted in Spanish because the verb conjugation implies the subject, yo is used for emphasis, contrast, or clarity. There are also a couple of key prepositions, entre (between) and según (according to), that are followed by yo instead of mí.
Yo como una manzana.
(I eat an apple.)
Mi hermano y yo fuimos al parque.
(My brother and I went to the park.)
Fui yo quien te llamó.
(It was I who called you.)
Ella es más alta que yo.
(She is taller than I am.)
Según yo, la película empieza a las ocho.
(According to me, the movie starts at eight.)
El secreto queda entre tú y yo.
(The secret stays between you and I.)
Aunque todos estén de acuerdo, yo no lo estoy.
(Even if everyone agrees, I do not.)
Yo pienso que es una buena idea.
(I think that it is a good idea.)
Mí
A1Mí is a prepositional pronoun. It is used when the pronoun I follows a preposition, such as a, para, de, en, por, sin, etc. It translates to me in English phrases like for me, about me, or from me. Mí is never the subject of a sentence. Note the crucial accent mark, which distinguishes it from the possessive adjective mi (my). A major exception is the preposition con (with), which merges with mí to form the special word conmigo (with me).
Este café es para mí.
(This coffee is for me.)
A mí me gusta mucho la música clásica.
(I really like classical music.)
No te vayas sin mí.
(Don't leave without me.)
Mi madre compró el coche por mí.
(My mother bought the car for me.)
Ellos siempre hablan bien de mí.
(They always speak well of me.)
¿Quieres venir conmigo al concierto?
(Do you want to come with me to the concert?)
Todos votaron en contra de mí.
(Everyone voted against me.)
Confía en mí, sé lo que hago.
(Trust me, I know what I'm doing.)
Summary
In essence, the choice depends entirely on the word's grammatical role. Use yo when I is the subject doing the action (I talk, I run). Use mí when I comes after a preposition like para, a, de, or sin (for me, to me, about me). Remember the main exceptions: use yo after entre and según, and use the unique word conmigo for with me.