日本の人口が少ないです。 when we define a subject 日本の人口, after the ga particle should there be always a adjective like 少ない for eg... can we place also place a noun ?
You name, it. It could be a noun, or a verb at least (or adverb(?)).
Noun: わだいになりました (became a (hot) topic)
Verb: へっています (is decreasing)
Adverb(?): ゆっくりへっています (is slowly decreasing)
p.s. I'm glad you are studying Japanese, I hope you work on it + progress!