Latin/Periods of Latin Learning
Which period of latin a person should learn to help learning other period of latin? Which period of latin should we learn to read latin literature and latin philosophy? What are the basic differences between ancient,classical and modern latin? From which period of latin most of the english vocabulary comes?
As a practical matter, almost all texts focus on the Classical Latin of the Golden Age, approximately 50 B.C. to A.D. 50. This is the period when the most influential literature was written in the language and includes authors like Vergil (epic poetry), Horace (lyric poetry), Cicero (oratory and philosophy), Julius Caesar (military history), Livy (history).
However, as your question indicates, the use of Latin is continuous to the present day, and there are many excellent works to read in all periods. However, the literature of the Golden Age remains the model for all subsequent periods, and the difference between periods, which is really not all that significant, is always described with reference to the Golden Age.
After the Golden Age comes the Silver Age of the later Roman empire, a particular favorite of mine, including such authors as Seneca (philosophy), Tacitus (history), and Suetonius (biography). If you read Classical Latin, you'll have no trouble with these. The Latin is a but terser, and there are only minor grammatical differences.
A bit later (4th century) comes St. Jerome's Latin Vulgate Bible, which thenceforth has a great influence on subsequent literature, particularly mediaeval literature, such as the religious and secular poets and the Scholastic philosophers like St. Thomas Aquinas. If you can read Classical Latin, Vulgate Latin, a simpler and more colloquial form, is a breeze.
With the Renaissance comes a look back to Classical Latin, and writers such as Plutarch model their style particularly upon Cicero. Modern Latin also follows the Classical models.
The Latin that forms some 80 per cent of English vocabulary come directly from the Classical period and a few centuries thereafter. Then English got a second infusion of Latin through French in the mediaeval period.
One of the best introductory Latin books for adults is Frederick Wheelock's "Latin." There are a number of supplemental works that key into it, and it has answers to the exercises in the back. I think that the most recent edition is overblown. It tries to do too much and can appear intimidating. You can get an earlier edition as a used book, which is more compact and to the point. This would be more in accord with Wheelock's original effective model.