Entre=between mi=mine Entre tú y yo=Between you and me It's not "ti y mi". Mi it's like "mine", so you can't use mi in that sentence.

You are right, Evelyn! The rule is that prepositions must govern a non-nominative case (oblique) and therefore the form to be expected is, indeed, "tí" and "mí". However, there is something VERY special about "entre": it is the ONLY preposition that requires THREE arguments (= "X is between Z and W", or "an X between Z and W"). All other prepositions take TWO arguments (X AT/ON/FOR/IN/FROM/TO.... Y). That creates a problem, because the structure of prepositional phrases provides only ONE subject and ONE COMPLEMENT slot, so the two arguments Z and W must be lodged into the unique complement slot following "entre". The only way to do that is to COORDINATE them and make them play the complement function together. That creates a structure ...X between [Z AND W], where "Z and W" form a coordinate phrase. Since case inflections can apply only to WORDS, but not to phrases, the preposition cannot impose its oblique case on the phrase [Z and W]. However, all 'personal pronouns' must appear with SOME case inflection, either the unmarked one (nominative: yo, tú, él, ella, nosotros, ellos) or a marked one (accusative/dative/genitive/oblique: me, mí, mi, te, tí, tu, les, los, las, su, se...). When there is no mechanism that enforces a marked inflection, THE UNMARKED one is automatically activated by default. That is what allows the nominative form "tú" and "yo" to appear after the preposition "entre" (exceptionally, to be sure, but subject to special rules). Well done! A good question! :-)