Medieval Latin had an enlarged vocabulary, which freely borrowed from other sources. It was heavily influenced by the language of the Vulgate, which contained many peculiarities alien to Classical Latin that resulted from a more or less direct translation from Greek and Hebrew; the peculiarities mirrored the original not only in its vocabulary but also in its grammar and syntax. Greek provided much of the technical vocabulary of Christianity. The various Germanic languages spoken by the Germanic tribes, who invaded southern Europe, were also major sources of new words. Germanic leaders became the rulers of parts of the Roman Empire that they conquered, and words from their languages were freely imported into the vocabulary of law. Other more ordinary words were replaced by coinages from Vulgar Latin or Germanic sources because the classical words had fallen into disuse.
Latin was also spread to areas such as Ireland and Germany, where Romance languages were not spoken, and which had never known Roman rule. Works written in those lands where Latin was a learned language, having no relation to the local vernacular, also influenced the vocabulary and syntax of medieval Latin.
Since subjects like science and philosophy, including Argumentation theory and Ethics (pre-law), were communicated in Latin, the Latin vocabulary that developed for them became the source of a great many technical words in modern languages. English words like abstract, subject, communicate, matter, probable and their cognates in other European languages generally have the meanings given to them in medieval Latin.
The influence of Vulgar Latin was also apparent in the syntax of some medieval Latin writers, although Classical Latin continued to be held in high esteem and studied as models for literary compositions. The high point of the development of medieval Latin as a literary language came with the Carolingian renaissance, a rebirth of learning kindled under the patronage of Charlemagne, king of the Franks. Alcuin was Charlemagne's Latin secretary and an important writer in his own right; his influence led to a rebirth of Latin literature and learning after the depressed period following the final disintegration of the authority of the Western Roman Empire.
Although it was simultaneously developing into the Romance languages, Latin itself remained very conservative, as it was no longer a native language and there were many ancient and medieval grammar books to give one standard form. On the other hand, strictly speaking there was no single form of "medieval Latin". Every Latin author in the medieval period spoke Latin as a second language, with varying degrees of fluency, and syntax, grammar, and vocabulary were often influenced by an author's native language. This was especially true beginning around the 12th century, after which the language became increasingly adulterated: late medieval Latin documents written by French speakers tend to show similarities to medieval French grammar and vocabulary; those written by Germans tend to show similarities to German, etc. For instance, rather than following the classical Latin practice of generally placing the verb at the end, medieval writers would often follow the conventions of their own native language instead. Whereas Latin had no definite or indefinite articles, medieval writers sometimes used forms of unus as an indefinite article, and forms of ille (reflecting usage in the Romance languages) as a definite article or even quidam (meaning "a certain one/thing" in Classical Latin) as something like an article. Unlike classical Latin, where esse ("to be") was the only auxiliary verb, medieval Latin writers might use habere ("to have") as an auxiliary, similar to constructions in Germanic and Romance languages. The accusative and infinitive construction in classical Latin was often replaced by a subordinate clause introduced by quod or quia. This is almost identical, for example, to the use of que in similar constructions in French.