Are there rules for the positioning of clitic pronouns?

Question

Are there rules for the positioning of clitic pronouns? Is there a preference between “poterlo dire” and “poter dirlo”, apart from how it sounds within a given sentence?

Answer 1

From the Academia della Crusca explanation:

There are five cases in which pronouns form a single word with the previous item:

1. with an imperative verb (e.g., dimmi, ascoltalo, parglagli)
2. with a gerund or in the past tense (e.g., vedendola, avendomi detto)
3. with a past participle with the function of the subordinate clause (e.g., giuntami notizia dell'accaduto, andai subito da lei)
4. with a present participle (almost only with the reflective one, e.g., confacentesi)
5. with an infinitive (e.g., andarci, proporgli). Just this case with the infinitive presents some peculiarities, one of which answers your question. When the infinitive is preceded by a modal verb (sapere, volere, potere, dovere, etc.), an unstressed pronoun might be combined with the action verb or with the modal verb (e.g., lo posso dire/posso dirlo). In many cases, the freedom of position of an unstressed pronoun allows using it both as a proclitic and as an enclitic (e.g., non riesco a vederlo/non lo riesco a vedere).

Besides, Enciclopedia dell'Italiano adds the following:

If a modal verb consists of two infinitives, you may have three choices: the pronoun can be in the initial position (lo devo poter fare), joined with the first infinitive (devo poterlo fare), or with the second infinitive (devo poter farlo).

Hence, both phrases in your question are grammatically correct.

Answer 2

The following explanation has been adapted from Grande grammatica italiana di consultazione. Vol. 1, pages 572-574, which is also where all the examples are from:

A clitic can move from the infinitive of the subordinate clause to the verb of the main clause (which is usually done in Tuscan and central-southern Italian) if

• the subject of the main clause is identical to the implicit subject of the subordinate clause and
• the verb of the main clause belongs to one of the following classes:

Modal Verbs

Mario vuole farlo da solo. / Mario lo vuole fare da solo. — Mario wants to do it on his own.

Antonio deve comprarlo. / Antonio lo deve comprare. — Anthony has to buy it.

Frederico può mangiarli. / Frederico li può mangiare. — Frederick can eat them.

Ugo sa prepararlo. / Ugo lo sa preparare. — Hugo knows how to prepare it.

The modal use of sapere in the last example needs to be distinguished from its non-modal use, in which it takes an infinitive complement introduced by the preposition di. The latter does not permit clitic climbing:

Ugo sa di comprarlo. — Hugo knows that he is buying it.

*Ugo lo sa di comprare.

Aspectual Verbs

La mamma comincerà a cucirla domani. / La mamma la comincerà a cucire domani. — Mom will start sewing it tomorrow.

Carlo finisce di dipingerlo oggi. / Carlo lo finisce di dipingere oggi. — Charles will finish painting it today.

Sandro continua ad amarla. / Sandro la continua ad amare. — Alex continues to love her.

Andrea sta per comprarla. / Andrea la sta per comprare. — Andrew is about to buy it.

Motion Verbs

Piero andò/venne/tornò a chiamarlo. / Piero lo andò/venne/tornò a chiamare. — Peter went/came/returned to call him.

Conative Verbs (Only Marginally)

Marco tenta di studiarlo. / ?Marco lo tenta di studiare. — Marcus is trying to study it.

Elio ha cercato di raggiungerlo. / ?Elio lo ha cercato di raggiungere. — Elliot has tried to reach it.

Anselmo prova a costruirlo. / ?Anselmo lo prova a costruire. — Anselm is trying to build it.

In the following examples, clitic climbing is not possible:

Aldo ha deciso di leggerlo. — Aldo has decided to read it.

*Aldo lo ha deciso di leggere.

Maria dice di scriverla domani. — Mary says to write it tomorrow.

*Maria la dice di scrivere domani.

Luisa ha promesso di spedirla. — Louise has promised to send it.

*Luisa la ha promesso di spedire.

Carlo vuole che lo compri. — Charles wants me/him/her to buy it.

*Carlo lo vuole che compri.