How to say "primarily" in Japanese

• 本来ほんらいJLPT N2approx.Common
  adverb, no-adjectiveMatches 'primarily' only in the sense 'essentially, fundamentally': what something is by its original nature. For the 'mostly, chiefly' sense it is not the word you want — use 主に or 主として.
  この詩しは本来ほんらいフランス語ふらんすごで書かかれていた。↗

  This poem was originally written in French.
• 主におもにJLPT N3approx.Common
  adverbThe straightforward word for 'primarily' meaning 'mainly, mostly, largely': it picks out the principal item, reason, or group among several. When in doubt for this sense, this is the safe choice.
  彼かれの病気びょうきは主おもに精神的せいしんてきな物ものだった。↗

  His illness was mainly psychological.
• 主としてしゅとしてCommon
  expression, adverbAlso covers 'primarily' as 'mainly, chiefly, for the most part', largely interchangeable with 主に for this sense. Either word is fine when you mean 'mostly'; this one simply leans a little toward 'chiefly'.
  これらの手紙てがみは主ぬしとして母ははから来きたものだ。↗

  These letters, in the main, are from my mother.

For 'primarily' meaning 'mostly, chiefly', use 「主に」 or 「主として」 (largely interchangeable); reserve 本来 for the 'essentially, at bottom, by original nature' sense.

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

• 本来ほんらいoriginally
• 主におもにmainly
• 主としてしゅとしてmainly