grand piano vs forte-piano

grand piano and forte-piano both are nouns.

Word	Noun	Adjective	Verb	Adverb
grand piano	Yes	No	No	No
forte-piano	Yes	No	No	No

As nouns, forte-piano is a hypernym of grand piano; that is, forte-piano is a word with a broader meaning than grand piano:

grand piano: a piano with the strings on a horizontal harp-shaped frame; usually supported by three legs
forte-piano: a keyboard instrument that is played by depressing keys that cause hammers to strike tuned strings and produce sounds

Other hypernyms of grand piano include piano, pianoforte.

grand piano (noun)	forte-piano (noun)
a piano with the strings on a horizontal harp-shaped frame; usually supported by three legs	a keyboard instrument that is played by depressing keys that cause hammers to strike tuned strings and produce sounds