An idiom or set phrase — a fixed expression whose combined meaning is not always deducible from the individual words.
英語の成句は直訳すると意味が分かりにくいことが多い。
English idioms are often hard to understand if translated literally.
辞書でその成句の用例を調べた。
I looked up examples of that idiom in the dictionary.
成句として定着している表現を丸ごと覚えると便利だ。
It's useful to memorize fixed expressions as whole idioms.
A proverb or saying; a concise, often traditional phrase that conveys a lesson or moral.
『石の上にも三年』はよく知られた成句（ことわざ）だ。
'Perseverance prevails' (literally 'three years on a stone') is a well-known proverb.
成句やことわざを日常会話に取り入れると表現が豊かになる。
Incorporating proverbs and idioms into daily conversation enriches expression.