In Minna No Nihongo II みんなの日本語初級II 文型練習帳(Exercise 38 on page 89) there is an exercise about のは, のが and のを, where a list of words is given, and you have to decide which words fit best with のは, which fit best with のが and which fit best with のを.
The question is:
「〜のは/〜のが/〜のを」といっしょによく使われることばをえらびましょう。
With the list of words being:
好きだ、危ない、忘れた、難しい、早い、遅い、体にいい、無理だ、上手だ、気持ちがいい、楽しい、知っている
The book already gives the answers for the first 2 as being:
のが好きだ
のは危ない
Can anyone explain why some of those words fit best with のは and some with のが? To my thinking, all of the words that could go with のは could alternatively go with のが, depending on the exact meaning you want to convey.
My understanding is that the の nominalizes the preceding phrase, and then the は or が are just the normal particles. So, のは would be used to show that the nominalized phrase is the 'topic' and のが would show it is the 'subject', irrespective of the actual word.
So, why does it suggest that some of those words fit best with のは and some with のが?