Question: What adjective (single word) means 'having/using a willingness to learn'?

Answer: 
How about *curious*?

> 
> * having a desire to learn or know more about something or someone
> * marked by desire to investigate and learn
> * eager to learn or know; inquisitive.
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Or inquisitive ^
Three excellent non-idiomatic words have already been suggested, curious, inquisitive, and teachable, but my intuition is that English lacks a word with your precise definition.

**I believe that the idiom "thirst" is what you are seeking.** "Thirst for knowledge" is a borderline cliche, but it is still evocative and clear, and I think many people would define it as a "willingness (or desire) to learn". (For the close connection between "will" and desire" see the interesting discussion, [“Make of that what you will” – meaning of “will”](https://english.stackexchange.com/questions/161230/make-of-that-what-you-will-meaning-of-will).)