When to use the verb "Ser" and when to use "Estar"

[Perikles]

I've just noticed this peculiar statement.

Originally Posted by pacomartin123

The close spelling of the Spanish verb "es" and the English word "is", is purely coincidence.

The online etymological dictionary gives for is:

third person singular present of be, O.E. is, from Germanic stem *es- (cf. O.H.G., Ger., Goth. ist, O.N. es, er), from PIE *es-ti- (cf. Skt. asti, Gk. esti, L. est, Lith. esti, O.C.S. jesti), from PIE base *es- "to be." O.E. lost the final -t-. See be.

This to me indicates that is and es are directly related, and is no coincidence. Moveover, to be and ser are the same verb. Estar derives from Latin stare., cognate with Greek istemi, meaning something like to set up.

Perhaps I'm missing something.