Of course tchrist is 100% correct about looking it up but I learned a rule-of-thumb in school that serves me well for the first vowel in a word:
If the first vowel is followed by a consonant and then another vowel,
the first vowel is long - as is the "u" in jupiter or tune
If the first vowel is followed by more than one consonant, the first
vowel is short - as is the "u" in rubber or rust
I'm sure there are many exceptions to this but it is a simple mnemonic tool that gives the speaker/reader some clue as to the pronunciation.
From The Families of Words, Mario Pei (Harper & Bros, 1962), page 11:
Even Anglo-Saxon, however, borrowed from the Latin and Greek of the missionaries who came to Christianize the heathen Saxon, or of the earlier Roman merchants who traded with the Germanic tribes while they were still on the European mainland......(church, (but this is from Greek circe), street, cheese (L. caseus), kitchen (L. coquina), mint, minster are a few"
On p.38, Pei adds "...cheap (Anglo-Saxon ceapian derived from Latin caupo "merchant") and shrive (Latin scribe)."
If these routes are interesting — traders with Germanic tribes and missionaries to the Anglo-Saxons — see Wikipedia, Latin Influence in English for many more examples, all of which are guaranteed pre-1066, but don't seem to be from the Roman occupation of Britain.
The Germanic tribes who would later give rise to the English language (the Angles, Saxon and Jutes) traded and fought with the Latin speaking Roman Empire. Many words (some originally from Greek) for common objects therefore entered the vocabulary of these Germanic people via Latin even before the tribes reached Britain (what is known as the Continental or Zero Period):
Cognates of virtually all of these English words exist in the other Germanic languages.
I put in the Latin for the words in this long Wikipedia list from The Families of Words and from Webster's New Collegiate Dictionary.
There is also a list of words from Wikipedia for the missionaries.
Since the Romans had occupied Celtic Britain before the arrival of the Anglo-Saxons, there are also words of Latin that came to English via Celtic. A source for these words is Latin through Celtic Transmission: Latin Influence of the First Period from Latin Influences on Old English by Albert C. Baugh & Thomas Cable. Although the Celts adopted at least 600 words from the Latin, very few of them survived after the arrival of the Germanic tribes — perhaps only five outside of elements in place names.
Best Answer
First of all, those statistics from Wikipedia may be a bit misleading, depending on your point of view. What they seem to have done is count every word in a 80,000-word dictionary once, regardless of whether the word is very rare or very frequent. Consider the preceding sentences:
I've marked the words of Latin or French origin with a plus sign, the others with a minus sign. That's 11 words of Romance origin out of 44 words, so 25 %. Note that regardless is dubious, because it is a word that French had borrowed from a Germanic language. Your average spoken English contains even fewer Romance words. The percentage of Germanic words generally goes up as the Romance percentage goes down.
The reason for this is that English has a very long tail of Romance words, but they are much less frequent on average than the words of Germanic origin. If you take a list of the most common words in English, they will be overwhelmingly Germanic.
As to the replacement of existing words, yes, sometimes they replaced existing Germanic words, but at other times they rather enlarged the vocabulary, especially the various technical and legal terms. The distinction is often difficult to establish, and not seldom meaningless, because many words become less frequent on their own account, so it is not always clear whether this is caused by pressure from a new word or just for no obvious reason.
The Romance words that came to England were mostly of a high register, as in law, rhetoric, philosophy, science, etc. There was the pressure from Latin on the one hand, the lingua franca across Europe, especially in writing; and from 1066 on, there was French, the language of the conquerors, who brought with them various elements of bureaucracy and high culture in French. It is usually the simpler, more frequent words that withstand such pressure the best, whence our continued attachment to words like is, the, a, he, she, do, be, have, etc.