When to use -ris vs. -re as a passive verbal ending

Question

Anyone who has read Cicero's famous line,

Quo usque tandem, Catalina, abutere patientia nostra?

...knows that the 2nd person singular passive personal ending "-ris" is often changed to "-re":

• laudaris > laudare
• hortaberis > hortabere
• mittaris > mittare
• audireris > audirere

Some of these--especially "mittare"--sound pretty strange to me. Others seem natural.

I have two related questions:

• Are there rules or guidelines, beyond obvious metrical constraints, about when to use either form?
• Do we have information about the relative frequency of -ris vs. -re?

Answer 1

I don't have precise figures for frequency, but G&L §1301b say that it is very common in earlier prose, less common in later:

In the second Singular, passive, in all tenses of the Present stem, the ending -re is much more common in early Latin than -ris, and is regular in Cic. except in the Pr. Indic., where he prefers -ris on account of confusion with Pr. Inf., admitting -re only in deponents, and then but rarely. In general, in the Pr. Indic. -re is rare in the first and second conjugations, more rare in the third, and never found in the fourth, in prose authors. Post-Ciceronian prose writers, e.g., Livy, Tacitus, prefer -ris, even in the other tenses of the Present stem. The poets use -ris or -re to suit metre.

For those unfamiliar with G&L's terminology, by "Present tenses" they mean all tenses formed from the present stem: for amare, that's am(a)- and not amav-.

For general advice on Latin composition, amabare is fine, but amare could cause confusion, so amaris is preferred. Note that even amere has been used in poetry, and although mittare has not, admittare and transmittare have.