I pronomi combinati
When you use a direct pronoun (lo, la, li, le) AND an indirect one (mi, ti, gli, le, ci, vi) in the same sentence, Italian smushes them together into a fixed pair: me lo, te lo, ce lo, ve lo, glielo. Two rules: (1) the indirect comes first; (2) mi/ti/ci/vi change their -i to -e before a direct pronoun. The 3rd person (gli/le) gets a special form: glielo β written as ONE word.
The vowel swap: i β e
When mi, ti, ci, vi are followed by a direct pronoun, the final -i turns into -e. So mi + lo = me lo, ti + la = te la, ci + li = ce li, vi + le = ve le. Why? Italian doesn't like two unstressed -i sounds in a row β the swap smooths it out. Maria mi dΓ il libro β Maria me lo dΓ = Maria gives it to me. Ti scrivo una lettera β Te la scrivo = I'm writing it to you.
The full grid
Indirect across the top, direct down the side. mi: me lo, me la, me li, me le. ti: te lo, te la, te li, te le. ci: ce lo, ce la, ce li, ce le. vi: ve lo, ve la, ve li, ve le. gli/le: glielo, gliela, glieli, gliele (ALL one word). Twenty combinations total β but they pattern. Just remember: indirect first, i β e, and gli/le merge into glie-.
Glielo, gliela, glieli, gliele β one word, two genders
Italian's most ruthless economy: gli (to him), le (to her), and modern gli (to them) ALL become glie- before a direct pronoun. So glielo can mean 'it to him', 'it to her', OR 'it to them'. Context disambiguates. Do il libro a Marco β Glielo do (I give it to him). Do il libro a Maria β Glielo do (I give it to her). Same form. Always written as ONE word β never two.
Where they go in the sentence
Same rules as single pronouns (lessons 13β14): before a conjugated verb, or attached to an infinitive/imperative. Me lo dai? = will you give it to me? Voglio dartelo = I want to give it to you (note: te lo + infinitive dare β dartelo, all one word, dropping the final -e of the infinitive). With negation: Non te lo dico = I'm not telling it to you.
With passato prossimo: agreement still applies
When a combined pronoun precedes a passato prossimo with avere, the past participle agrees with the DIRECT pronoun (lesson 16's rule still holds). Me l'ha dato (the lo agrees β masculine sing.). Me l'ha data (la β feminine sing.). Me li ha dati (li β masc. plural). Me le ha date (le β fem. plural). Glielo, gliela, glieli, gliele trigger the same agreement: Gliel'ho detta (the news, fem.).
