One's former grade or class (the grade previously held, e.g. when returned to an earlier year/class).
試験に落ちたため、彼は原級に戻された。
Because he failed the exam, he was returned to his former grade.
その学校では習熟度に応じて原級で再学習させることがある。
At that school, students are sometimes made to re-study at their former grade level according to proficiency.
In grammar, the positive degree (the base form) of an adjective or adverb, as opposed to comparative or superlative (e.g. 'big').
『big』は原級で、『bigger』は比較級、『biggest』は最上級だ。
'Big' is the positive degree, 'bigger' is the comparative, and 'biggest' is the superlative.
形容詞の原級は比較の基準になるため、文脈で意味が変わらないか確認する。
Since the positive degree of an adjective serves as the comparison baseline, check in context whether its meaning changes.
英語学習では原級・比較級・最上級をまとめて覚えると便利だ。
When learning English, it's useful to learn the positive, comparative, and superlative together.