When do verbs end in ar/er/ir instead of are/ere/ire? [duplicate]

Question

I've been studying Italian for a few months, and I've learned that verbs end with are, ere or ire. However, I've started reading some Italian and I've seen many cases where the "e" seems to be dropped. I understand cases such as

dargli
ribellarsi

because the e is replaced by the direct object. But why is the "e" dropped in cases like

Come poter dar torto a ...

I would have expected the above to be

Come potere dare torto a ...

and I would really like to understand the rule for when the following "e" is dropped.

Answer 1

It's called "troncamento" in Italian (and it's not limited to verbs, compare "dottor Rossi").

This may provide some guidance: http://www.treccani.it/enciclopedia/troncamento_%28Enciclopedia_dell%27Italiano%29/

Note section 2,

Tra una forma verbale e la parola successiva il troncamento può verificarsi, purché le due parole siano strutturalmente vicine, e quindi appartengano allo stesso gruppo intonativo (➔ pronuncia; ➔ prosodia). Questo è il caso, ad es., di parlar chiaro, vuol fare, andiam bene, venivan tutti. Non c’è cancellazione invece in *deve partir – se non sbaglio – domani, in cui se inizia un nuovo gruppo intonativo costituito dalla frase parentetica (cfr. Nespor 1993).

Basically, in the case of verbs it is optional and used for euphonic and economical reasons inside a tone group - provided examples convey the idea quite well, most importantly: avoid it in front of parentheticals.

Note that - according to the link and to my experience - it doesn't always sound "right" to native ears and, especially with overuse, has a high risk of sounding affected (it is a stereotypical feature of older Italian in movies and such) or plain weird/wrong, while only in extremely rare examples (mostly shibboleths and sayings) not using it doesn't sound right.

Troncamento in verbs is also often used to avoid a spurious rhyme or consonance which could sound just a bit funny, as it's precisely the case with "potere dare" in your example - "come poter dar torto" is a bit of a set phrase as well (see above); note that "potere dare" is generally acceptable, though.

For the above reasons, beginner-to-intermediate speakers should best avoid it.