No single person invents a language, it is spoken by thousands, sometimes millions of people. In primitive times people learned the sounds their parents customarily associated with words and ideas, they didn't give much thought to it, and wouldn't really notice the small changes each generation gave to speech. People would wander around and be separated from each other, divided by uncrossable rivers or mountain chains, and would start to speak differently.
Sometimes people would come together, in peace or in war, and new words would be exchanged.
The surviving language closest to English, I heard, was Frisian, spoken around the borderlands between Germany and Denmark. From that area, 1,500 years ago, the Angles and Saxon tribes invaded England and displaced the Celts, who themselves were somewhat Romanized and adopted some Latin words.
There were many more languages in the Medieval times when travel was difficult and people separated by 50 miles could end up speaking differently. Shifts in pronunciation occurred, and shifts in the uses we put to words.
For example, Nicole Breit mentioned that "knife" is now pronounced with a silent k but in the old Anglo-Saxon language it WAS pronounced. The word even got borrowed into French. The USUAL word for knife in French is "couteau" and that's the basis of the English word "to cut" -- but in French a pen-knife or pocket-knife is a "canif" (pronounced kaa-nif,) which was a borrowing from the way the Germanic tribes pronounced their word for "knife" at the time, and preserves the original K sound, unlike what happened in English!
Of course, even in English, people from different parts of the world pronounce "knife" differently and have different ideas of what a knife should be: