The book The Unfolding of Language[1] describes the forces that shape how language evolves. One is extending the use of a pattern (e.g. "en" for plural), even in cases like this one where it wasn't technically appropriate. Another is the use of metaphor. E.g. "discover" used to mean "to remove the cover of", but now its meaning is purely metaphorical and the literal meaning has been mostly lost. Another is laziness: slurring long compound phrases together until they're effectively one word. A lot of conjugations/declentions are a result of this. I recommend this book if you're interested in how languages change over time; it's very well written.

EDIT: Another fun fact is that words sometimes begin to mean their _exact opposite_. For example, "wicked" used to mean "evil", but in England (and elsewhere, but especially England) it's started to mean "sweet".

And there's always the great consonant shift: https://en.wikipedia.org/wiki/Grimm%27s_law

[1] https://www.amazon.com/Unfolding-Language-Evolutionary-Manki...

An interesting read on the subject: http://www.amazon.com/The-Unfolding-Language-Evolutionary-In...