How to say "voluntarily" in Japanese

• 進んですすんで
  adverbCovers the positive sense: doing something willingly, of your own free will, because you want to. Typical when you offer help or take on a task no one assigned, gladly and without being persuaded.
  進すすんで学まなぼうとする気きの無ない者ものには教おしえる甲斐かいがない。↗

  One who is not willing to learn is not worth teaching.
• 自分からじぶんから
  expressionEmphasizes acting of one's own accord, on one's own initiative, without being told or prompted. Suits contexts like apologizing or speaking up before anyone asks; the accent is on initiative, not eagerness.
• 勝手にかってに
  adverbMainly covers acting arbitrarily, without asking, doing as one pleases, often with a critical nuance; also for things acting of their own accord. For willing human action it is seldom the word you want.
  書類しょるいは勝手かってにいじられた形跡けいせきがあった。↗

  The documents were tampered with.

Use 「進んで」 for doing something willingly and gladly of your own free will, 「自分から」 for acting on your own initiative without being told, and 「勝手に」 mainly for acting arbitrarily, without asking.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 進んですすんでvoluntarily
• 自分からじぶんからwillingly
• 勝手にかってにarbitrarily