How do we know the events described in the bible are true? Many Christians will quote passages from the bible as proof or evidence when needed. However, given that it is a story written, translated, interpreted, re-written, re-translated and re-interpreted by humans over tens of generations and adapted for many of the monotheistic religions such as Christianity, how can we be sure that it still accurately represents the original material, even if it were written by a monotheistic god?

A commenter has made a good point about there being very old manuscripts presumably containing similar information to modern interpretranslated bibles, though this raises more questions for me: 


Are those very old manuscripts written in God's handwriting?
Is there anyone alive who knows the language they were written in, and hasn't needed to try and interpret what they say by cross-referencing various documents of a similar age to find patterns and derive meanings, essentially using a bit of detective work and some educated guesses?
I'll post an answer to one aspect of your question "How do we know that the Bible hasn't changed since it was written?"

It all boils down to manuscripts.

The number and age of the manuscripts that we have shows that nothing of significance has changed since the earliest Christian times.

Here is a list of some of the earliest manuscripts.  For reference on this site:

Author  Date Written            Earliest Copy   Time Span   Copies (extent)
Magdalene Ms (Matthew 26)       1st century     50-60 AD    co-existant (?)  
John Rylands (John)             90 AD           130 AD      40 years     
Bodmer Papyrus II (John)        90 AD           150-200 AD  60-110 years     
Chester Beatty Papyri (N.T.)    1st century     200 AD      150 years    
Diatessaron by Tatian (Gospels) 1st century     200 AD      150 years    
Codex Vaticanus (Bible)         1st century     325-350 AD  275-300 years    
Codex Sinaiticus (Bible)        1st century     350 AD      300 years    
Codex Alexandrinus (Bible)      1st century     400 AD      350 years    


Through these (and many other) manuscripts, we have been able to show that the Bible of today has not been altered at since its earliest manifestation.