Old French (ancien français, in French), is a general term for the various langue d'oïl dialects that prevailed in northern France from about C.E. 1000 until approximately 1300. In many modern texts, though, it's used in reference to what linguists have named francien, the dialect of the Ile-de-France. (This dialect would have been known simply as français in the Middle Ages.) This retroactive standarization is from a modern viewpoint; it does not reflect a medieval standard, which didn't exist. If we think of the evolution of French as comprising the stages Vulgar Latin > Gallo-Roman > Old French > Middle French > modern French, it represents the earliest point which can be called "French."
Old French is the result of the admixture of many languages and cultures, and of all the Romance languages, French is the most changed from Latin. In Gaul, Vulgar Latin was imposed on the Celtic inhabitants by occupying and colonizing Romans, most of whom were probably illiterate. The pronunciation, grammar, and vocabulary of Vulgar Latin was not insignificantly different from Classical Latin. Furthermore, the natural evolution of Latin in Gaul was greatly affected by the invasions of the Germanic tribes in late Antiquity and by the Normans in the early Middle Ages, bringing vocabulary and sounds unknown in other Romance languages today.
The phonology of Old French is more complicated than either modern French or Latin. French in general has more phonemes than Latin, but the large number of Old French phonemes have been streamlined in modern French, particularly the vowels. Old French had several diphthongs and tripthongs, which surivive today only in spelling; words such as beau and faire were not pronounced with the simple vowels of today, but as /biau/ or /bjau/ and /fair@/. Nasalized vowels occurred before any nasal consonant, and both the nasal vowel and the nasal consonant were pronounced. Compare OFr bon /bo~n/ and bone /bO~n@/ with modern French bon /bo~/ and bonne /bOn/.
Unlike Modern French, but like Latin, Old French maintained distinctive noun cases. The Latin system of six (five) cases had tapered off to two cases by 1000, the subject case and the oblique case (cas-sujet and cas-régime). The subject case, the descendant of the Latin nominative (usually), was used as grammatical subject and its attributes. The oblique case (descended from the Latin accusative) took all other functions, including sometimes the Latin genitive.
Old French masculine nouns followed three declension patterns. The first pattern applied generally to nouns whose nominative singular ended in -s, the second to nouns which did not, and the third to nouns which were imparisyllabic, mostly descended from third-declension Latin nouns. Here is a paradigm of li rois (the king):
Nominative singular: - li rois
Oblique singular: - le roi
Nominative plural: - li roi
Oblique plural: - les rois
The verb system is very close to Modern French, in terms of the types and structures of tenses. (Most of the changes in verb morphology have been due to phonetic evolution.) Every tense that exists in Modern French exists in Old French, but some of the uses are different. The passé simple, imperfect subjunctive, and other literary tenses are much more frequent. As in Spanish and Italian (and Latin), subject pronouns are unnecessary, endings indicating person. Here follows an example conjugation. Note the lack of ending on the first-person singular. Also, the ending of the third-person plural was already silent by this time.
The irregular verb estre:
The literature of Old French comprises several genres; the Chansons de geste (La Chanson de Roland), the Romances or Littérature courtoise (Le Roman de la rose, Tristan et Iseult), popular literature (Le Roman de Renart), chronicles (Villehardouin, Joinville, theater (La Farce de maistre Pathelin), and poetry (Guillaume de Lorris). Not all the literature is written in Francien. These represent a period of almost 500 years, and there is a vast different between the language of the early works and the later ones.
The study of Old French is important not just for understanding the history of the French language, but for English as well. It was the Norman dialect which influenced English so much after 1066, and many English words resemble Old French more than they do modern French. Take the word faith:
The last step represents the state of the word in the 12th century, and it was pronounced /fOi/. (At this point the system of writing stopped changing, although the pronunciation continued to evolve: /fOi/>/fOE/>/fwE/>/fwa/.) The word carried into England by the Normans was probably the third from the end, pronounced /feiD/ or /feiT/.