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Why Spanish says 'entre tú y yo' and 'según tú'

After most prepositions, Spanish swaps yo and tú for special forms: para mí, de ti, sin mí, conmigo. A small group of words breaks this rule and keeps the ordinary subject forms instead. You say según tú (according to you), entre tú y yo (between you and me) and todos menos yo (everyone except me). The words excepto, salvo, hasta and incluso work the same way.

English speakers trip over this in two ways. Once you have learned para mí, it feels natural to say entre ti y mí, which sounds wrong to native ears. English also pulls the other way: you say "between you and me" and "except me" with object forms, so the Spanish yo seems backwards at first.

Word list
entreA1 between entre tú y yo = between you and me entreA2 among entre tú y yo = between the two of us (together) exceptoA2 except excepto yo = except me menosA2 except menos yo = except me salvoA2 except salvo tú = except you aunB1 even aun yo = even I (aun without accent means 'even') hastaB1 even hasta yo = even I inclusoB1 even incluso tú = even you segúnB1 according to según tú = according to you
How it works

The normal pattern is preposition + mí or ti. With con, these combine into conmigo and contigo:

Este regalo es para ti, y aquel es para mí.
(This present is for you, and that one is for me.)

The exception words ignore that pattern and take yo and tú, the same forms you use as the subject of a verb. The other pronouns (él, ella, nosotros, ustedes and so on) look the same in both patterns, so the difference only shows up in the first and second person singular.

Rule of thumb: if the word means according to, between/among, except or even, use yo and tú. Never use mí, ti, conmigo or contigo after them.

The words
According to: según

Según introduces whose opinion or information you are reporting.

Según tú, todos se equivocan menos yo.
(According to you, everyone is wrong except me.)
Between and among: entre

Entre usually links two people with y, and both pronouns take the subject form: entre tú y yo, never entre ti y mí. Think of the pair as one joint subject.

Esto queda entre tú y yo.
(This stays between you and me.)

With the meaning "among" or "together", entre shows that people share a task:

Entre tú y yo podemos pintar la casa en un día.
(Between the two of us, we can paint the house in a day.)
Except: excepto, salvo, menos

These three mean the same thing. Menos is the most conversational and salvo is slightly more formal.

Todos fueron a la fiesta excepto yo.
(Everyone went to the party except me.)
Todos están de acuerdo salvo tú.
(Everyone agrees except you.)
Todos llegaron a tiempo menos yo.
(Everyone arrived on time except me.)
Even: hasta, incluso

When hasta and incluso mean "even" (something surprising), they also take yo and tú.

Es tan fácil que hasta yo lo entiendo.
(It's so easy that even I understand it.)
Incluso tú te reíste con esa película.
(Even you laughed at that film.)
Watch out
When the verb needs a

Some verbs take a before a person. If you repeat that a after excepto, salvo or menos, the normal rule returns and you need mí or ti, because mí now follows a:

Nos invitaron a todos excepto a mí.
(They invited all of us except me.)
Hasta meaning "up to"

When hasta means "as far as" or "until", it behaves like an ordinary preposition: la noticia llegó hasta mí (the news reached me). Only the "even" meaning takes yo and tú.

Según yo

It is grammatical and common in Latin America, but it can sound colloquial. Many speakers prefer en mi opinión or para mí when giving their own view. Según tú is neutral everywhere.

The typical mistake is carrying over the mí habit: entre ti y mí or menos mí. If the word is on this short list, switch back to yo and tú.

Summary

Most prepositions take mí and ti, but según, entre, excepto, salvo and menos, along with hasta and incluso meaning "even", take yo and tú: según tú, entre tú y yo, todos menos yo, hasta yo. The one exception is when an extra a comes in, as in excepto a mí.