Breakdown of Поначалу мне казалось, что эта книга скучная, но вскоре я изменила мнение.
Questions & Answers about Поначалу мне казалось, что эта книга скучная, но вскоре я изменила мнение.
Поначалу means at first / initially. It often suggests that the situation later changed, which fits this sentence well: first the speaker had one impression, then changed it.
Compared with сначала:
- поначалу = at first
- сначала = at first / at the beginning / first
In many contexts they overlap, but поначалу very often sounds especially natural when followed by a contrast like но потом / но вскоре.
Because казаться here means to seem (to someone), and Russian uses the person experiencing that impression in the dative case:
- мне казалось = it seemed to me
If you say я казалась, that means I seemed or I appeared, so я would be the thing giving the impression, not the person having it.
So:
- мне казалось, что... = it seemed to me that...
- я казалась... = I seemed...