In early Italian, the use of various forms of the definite article was somewhat different than today. The form lo was more frequent than in modern Italian, and it was also used in many cases in which il was subsequently called for. Today lo precedes nouns beginning with s impura (s + consonant), (lo Stato), z (lo zio), gn (lo gnomo), sc (lo sciocco), pn (lo pneumatico), ps (lo psicologo), x (lo xilofono), and with i semiconsonantica (semivowel i) (lo iodio). All other masculine nouns starting with a consonant are preceded by the article il. In early Italian, however, the form il could only be used after a word ending in a vowel and before a word beginning with a consonante semplice (simple consonant). In those cases it could also occur in the reduced form 'l. Here are two examples from Dante's Divine Comedy (more specifically from Inferno: Canto I:
m'avea di paura il cor compunto (verso 15);
là, dove 'l sol tace (verso 60).
However, the form lo can be used in both cases, given that the final sound of the previous words end in vowels and the initial sounds of the next words end in simple consonants. In particular the use of this form was mandatory at the beginning of a phrase. Here are some examples, again taken from Dante's Divine Comedy:
si volse a retro a rimirar lo passo (Inferno: Canto I, verso 26);
Tu se' lo mio maestro (Inferno: Canto I, verso 85);
Lo giorno se n'andava (Inferno: Canto II, verso 1).
The differences in the use of the articles lo and il could be summarized as follows: in early Italian, lo was used more frequently and could be used in all cases (even if il was expected). In modern Italian il is found more frequently, and unlike in early Italian there is no overlap in usage of the two articles.
The early use of the article lo instead of il continues in contemporary Italian in adverbial phrases such as per lo più (for the most part) and per lo meno (at least). Another form which still occurs today (but in very limited use), is the plural li. This form is sometimes found when indicating a date, especially in bureaucratic correspondence: Rovigo, li marzo 23 1995. Since li isn't an article recognized by most Italians today, it is not uncommon to see it misspelled with an accent, as if it were the adverb of place lì. Of course, when speaking one says Rovigo, il marzo 23 1995, while in general in correspondence it is preferred to write 23 marzo 1995 (without the article).
