Topic particles


> ボブ：アリスは学生？  
>  Bob: Is Alice (you) student?
>
> アリス：うん、でもトムは学生じゃない。  
>  Alice: Yeah, but Tom is not student.

In this example the topic is about students. Why do you have to say _student_
a second time right here ”トムは学生じゃない”




First, please make sure what a grammatical _topic_ is. It is true that the
theme of this conversation is "who are the students?", but that's not a
grammatical topic of these sentences.

> アリスは学生？  
>  うん、でもトムは学生じゃない。

Here, the topic of the first sentence is アリス, and the topic of the second
sentence is トム, because they are clearly marked with the topic marker, は.
学生だ/学生じゃない are called a predicate of a sentence.

In Japanese, you can omit the topic altogether if it can safely be inferred
from the context. In this case, however, you can't omit トムは because the topics
of the two sentences are different.

Second, you may be interested if it's possible to avoid saying 学生 twice
anyway. In English, you can often omit predicates to avoid repetition, like
this:

> Are you a student? -- Yes I am _(a student)_ , but Tom is not _(a student)_.

You can avoid saying 学生 twice in Japanese, too, but it's a bit trickier. You
can **not** simply drop the repeated word and say トムはじゃない or トムはない. Instead,
you have to say something like 私はそうですが、トムは違います。

