a subject of conversation 话题
a subject of research 研究的题目
What is the subject for today's discussion? 今天的讨论题目是什么?
Love and death are the subject of the poem. 爱与死是那首诗的主题。
(以上摘自外研社、建宏《英汉多功能词典》p.1490)
The main theme of the play was clear. 该剧总的主题是清楚的。
Love is a favourite theme for poetry. 爱情是诗歌里喜欢用的主题。
(以上摘自《薄冰英语惯用词典》)
2. topic 后面的介词一般用 of ,例如: