1. 나는 영어가 좋아.
In this sentence, what is the subject? '나'? And what is the subject particle? '는'?
In the Korean language, the subject particles are '이/가', '께서' and '에서'.
'은/는' is not subject particles, it is called auxiliary particle. I think this is controversial, but anyways it is the authentic Korean grammar.
Therefore it is not easy for me to answer for your question "I learned that we may or may not attach the subject/object particles to "만" as in 나만 영어가 좋아 / 나만은 영어가 좋아.Is there a difference in nuance between them or are they interchangeable? "
2. 나만 영어가 좋아 / 나만은 영어가 좋아
'나는 영어가 좋아.' This kind of sentence is called '이중주어문' by some grammarians, which means the sentence has two subjects. Let's subdivide this sentence into two sentences having its own subject. '나는 좋아.' And '영어가 좋아.' The fact that the predicate in this sentence is a adjective makes it tricky to explain. I will take another example.
'그는 영어를 좋아한다.' ---'는' topic subjective article. If/'when I talk about him, I say he likes English. I do not know about other people and am not including them in the area of a topic.
'그만 영어를 좋아한다.'--- He likes English, whereas other people do not.
'그만은 영어를 좋아한다.'-I strongly emphasize the fact that he is the only one who likes English. The topic is restricted to him only and the fact that he likes English is emphasized.
3. '공부만 해요 / 사랑만 해요 / 후회만 해요 '
'나는 한국어를 좋아한다.'
'나는 한국어만을 좋아한다.' (I do not like math, chemistry.....)
'나는 한국어를 좋아하기만 한다.'(I like Korean very much but I do not like/actually hateㅋㅋ/ learning, studying, memorizing words, and ......)
The Korean Grammar says that 1) when attaching to nouns, 만 usually attaches to subjects or objects, in which case it replaces the subject and object particles, 2) 만 can also attach to other particles, adverbs and verb endings.