Personal pronouns in Spanish (‘yo’, tú’, ‘ella’, etc.) are commonly used after prepositions (‘de’, ‘para’, ‘por’, etc.). While most of them remain the same when used like this, ‘yo’ and ‘tú’ change to ‘mí’ and ‘ti’ respectively (note that ‘mí’ has an accent and ‘ti’ does not). So we say ‘Esta es una carta para ellos’ = ‘This is a letter for them’, BUT ‘Esta es una carta para ti’ = ‘This is a letter for you’. This happens for all prepositions EXCEPT the following ones: ‘entre’, ‘excepto’, ‘incluso’, ‘salvo’, ‘según’ and ‘menos’.