How to say "mainly" in Japanese

• 主におもにJLPT N3approx.Common
  adverbThe default, all-purpose word for 'mainly' or 'primarily'. Use it whenever something is done chiefly for one reason or concerns mostly one thing, with no special nuance attached.
  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.
• 大概たいがいJLPT N1approx.Common
  adverbCovers 'mainly' only in the loose sense of 'generally, usually, normally'. When English 'mainly' just means something happens as a rule, this fits; for precise 'primarily', another word is better.
  彼かれは夜よるにはたいがい家いえにいる。↗

  He is generally at home in the evening.
• 主としてしゅとしてCommon
  expression, adverbA slightly more formal written-style equivalent of 'mainly; chiefly; primarily', suited to essays, reports, and explanations where you state what something mainly consists of or is aimed at.
  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.
• 大半たいはんJLPT N3approx.Common
  noun, adverbBest when 'mainly' means 'the greater part of' something concrete — most of a group, a period, or a quantity — i.e. 'mostly; largely; predominantly' rather than a stated primary purpose.
• もっぱらJLPT N1approx.Common
  adverb, no-adjectiveNormally written in kana alone; conveys 'principally; exclusively' — activity or attention devoted to one thing. Suits 'mainly' when the sense is 'devoted solely to', and is a poor fit for casual 'mostly'.

Choose 「主に」 or 「主として」 for neutral 'mainly/primarily', 大半 for 'the majority of', 「もっぱら」 for exclusivity, and 大概 when 'mainly' blends with 'usually'.

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

• 主におもにmainly
• 大概たいがいgenerally
• 主としてしゅとしてmainly
• おおむねin general
• 多くおおくmostly
• 大半たいはんmostly
• もっぱらprincipally