I've tried looking this up in every grammar guide I can find, but I haven't found the answer. Consider these two sentences: I bought a car to drive myself to work. I bought an alarm clock to wake me in the morning.
For my birthday, I received Don Ringe's From Proto-Indo-European to Proto-Germanic and I've been reading it while taking the train in to work.
I find that even though I'm not in academia any more it's refreshing to spend some time perusing hard-core historical linguistic geekery, particularly since I've never really delved into PIE with the depth that I should have. I suspect that many Anglo-Saxonists tend to skimp on their study of the linguistic pre-history of English, especially the earliest stages.