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To compose or write the lyrics (words) of a song
彼女は毎日歌詞を書く時間を取っている
She sets aside time every day to write lyrics
この曲の歌詞を書いたのは有名な作詞家です
The lyrics for this song were written by a famous lyricist
This phrase specifically refers to the process of creating the textual content of a song, distinct from composing the melody.
Avoid confusing 歌詞を書く with 曲を書く (to compose music).
Combination of 歌詞 (lyrics) and 書く (to write)
This phrase is commonly used by musicians, lyricists, and songwriters. It can refer to both professional and amateur lyric writing.