Classical Chinese: it is the written language used by ancient people in ancient grammar habits;
Poetry: In the early stage of the formation of poetry, poetry and song are inseparable. However, because the lyrics were recorded and preserved in the form of words, and because the music score was not perfect at that time, the songs could only be circulated orally. Later, due to the lack of effective carrier preservation, the songs were gradually lost. The lyrics that lost points became later poems. For example, The Book of Songs is a poem. After losing the score, all you can see now is the lyrics. There is also Song of the Wind. Later people can only see the lyrics and can't sing. In particular, the introduction of Buddhism brought Sanskrit and the notation of scales. Poetry and song gradually began to separate. After Sui and Tang Dynasties, poetry and song were almost completely separated. Especially after the rise of ci, poetry completely lost its original function as a song, but only existed as a verse.
Speaking of which, let's talk about piano music. Before the musical score of Miyachi appeared, there was a piano score. Piano scores are written in special Chinese characters, but they are abstruse and irregular. Most piano scores are songs without words.
Poetry: Most people put poetry and words together. In fact, the two have deep roots. At first, words can be said to have developed from poetry, so words are also called poems. But I personally think that ci developed from poetry and appeared in the Tang Dynasty. In fact, words are written according to a certain tune, and a tune can have different words. So the word song is more appropriate.
Qu: The earliest songs should be music without lyrics. Later, when it came to the so-called Yuanqu in the Yuan Dynasty, I thought it was a traditional opera.