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...

With verbs like казаться, Russian often marks the experiencer with the dative:

• мне = to me
• ему = to him
• нам = to us

So мне казалось is literally something like to me, it seemed.

This pattern is very common in Russian:

• мне нравится = I like
• мне кажется = it seems to me
• мне хотелось = I wanted / I felt like

English usually uses a subject like I, but Russian often uses dative in these meanings.

Because this is an impersonal use of the verb. In the past tense, Russian often uses the neuter singular form for impersonal statements:

• казалось
• хотелось
• думалось
• удалось

So казалось here does not agree with мне. It is just the standard past-tense form used in impersonal expressions, similar to English it seemed.

Because скучная agrees with книга:

• книга = feminine singular nominative
• скучная = feminine singular nominative

This is the normal form when the adjective is part of the statement the book is boring.

The form скучной would usually appear in a structure with была:

• эта книга была скучной = this book was boring

So:

• эта книга скучная = this book is boring / this book is a boring one
• эта книга была скучной = this book was boring

In your sentence, the speaker is reporting the content of a past impression, and Russian very naturally uses что эта книга скучная in that kind of context.

Russian often leaves out the verb to be in present-tense-style statements:

• книга скучная = the book is boring

After verbs like думать, считать, казаться, Russian often keeps that same structure when reporting what someone thought or felt:

• Я думал, что фильм интересный
• Мне казалось, что книга скучная

Even though the main verb is in the past, the content of the thought can still be phrased this way.

A more explicit version is also possible:

• Мне казалось, что эта книга была скучной

That version sounds a bit more clearly tied to the past. The original sentence sounds very natural and conversational.

Because it agrees with книга, which is a feminine noun.

Russian adjectives must match the noun in gender, number, and case:

• скучный фильм = boring film
• скучная книга = boring book
• скучное письмо = boring letter
• скучные книги = boring books

So here:

• эта книга = feminine singular
• скучная = feminine singular

Both can mean soon, but вскоре often means soon afterwards / before long, with a clearer connection to what came before.

Here the sentence says:

• first the speaker thought the book was boring,
• but soon after that she changed her opinion.

That makes вскоре a very good fit.

Roughly:

• скоро = soon
• вскоре = soon afterwards / before long

Because the sentence describes one completed change of opinion, so Russian uses the perfective verb:

• изменить → perfective
• изменила = changed

The imperfective изменяла would suggest something ongoing, repeated, or process-focused, which does not fit as well here.

Compare:

• Я изменила мнение = I changed my mind/opinion
• Я часто изменяла мнение = I often changed my mind

So the perfective is the natural choice here.

In the past tense, Russian verbs agree with the speaker's gender if the subject is singular.

So:

• я изменил = a male speaker
• я изменила = a female speaker

This sentence tells you that the speaker is female.

Because in Russian, the possessive word is often omitted when it is obvious from context.

So:

• я изменила мнение naturally means I changed my opinion / I changed my mind
• я изменила своё мнение is also correct, but it adds emphasis or explicitness

Russian often leaves out words like my, your, his/her when they are already clear.

Yes. Literally it means to change an opinion, but in many contexts it works very much like English change one’s mind.

A small nuance:

• изменить мнение often focuses on a judgment or opinion about something
• передумать often means to change one’s mind about a decision or plan

For example:

• Я изменила мнение об этой книге = I changed my opinion about this book
• Я передумала идти = I changed my mind about going

In your sentence, изменила мнение is exactly the right expression.

Yes. Russian word order is flexible, and the sentence could be rearranged without changing the basic meaning.

For example:

• Поначалу мне казалось...
• Мне поначалу казалось...

Both are natural.

The original order puts поначалу right at the start, which highlights the time frame: at first. That works well because the sentence then contrasts it with но вскоре.