Final answer:
The subject of the sentence is "la mejor amiga de mi hermano" which means "my brother's best friend." This is a grupo nominal indicating the person who is always seeming sad is the best friend of the speaker's brother.
Step-by-step explanation:
The sentence "La mejor amiga de mi hermano siempre parece triste" is in Spanish and translates to "My brother's best friend always seems sad." The subject of the sentence is "la mejor amiga de mi hermano" which means "my brother's best friend." This is a grupo nominal or a noun phrase, indicating the person who is always seeming sad is the best friend of the speaker's brother.
Learn more about Subject in Spanish sentence