In their discussions of the formation of the infinitive, both Pharr and White remark on the contraction of -εεν to -ειν, but I don't understand why this would apply to most of the examples that come out to be -ειν. White offers the specific example of λύω, showing the formation of its infinitive as λυε+εν -> λύειν. But I don't understand where the extra ε even comes from. The stem is λυ, not λυε.
I spun this question off, as suggested by cmw, from a longer question that was unwieldy.