I'm pretty sure you mean
공부한다 to study
so the "dictionary form" is
공부하다. from there it can be conjugated many many different ways, but let me focus a minute just on why the ㄴ다 was added.
Verbs are broken down into two main categories, as shown by the headers below.
The main conjugation into an informal, lacking polite form version adds ㄴ or 는 depending if the final sound is a vowel or not. Like this:
Dictionary form (minus 다) +
In the case of
공부하 + we must use the ㄴ by itself. If you were to conjugate the dictionary form of "to eat", the final sound is a consonant, so 는 is used instead.
먹다 (dictionary form) = 먹는다
The main conjugation of a descriptive verb into an informal, lacking polite form version adds...nothing. Like this:
So in the case of
예쁘다 - it can just be used as is, straight out of the dictionary:
예쁘다 (dictionary form) = 예쁘다
Other conjugation examples of polite forms include:
공부하다 (dictionary form)
- 공부하세요? Are you studying? (Do you study?)
- 공부해요. I'm studying. (I study.)
- 공부하십니까? Are you studying? (Do you study?)
- 공부합니다. I'm studying. (I study.)
- 예뻐요? Is it pretty? (Am I pretty? Is she pretty? etc)
- 예뻐요. It is/she is/he is/I am/you are pretty.
- 예쁩니까? Is it pretty? (Am I pretty? Is she pretty? etc)
- 예쁩니다. It is/she is/he is/I am/you are pretty.
나는 책을 공부한다
is grammatically correct (and maybe more natural) and
나는 책을 공부해
is grammatically correct (but probably needs the 요 to be natural...depending on context and emotion).
Also...you can study the book, but "read" the book will trump the study in a frequency contest:
나는 책을 읽는다 (읽다 dictionary form of "to read").