Без мотива следовательница вряд ли поверила бы в такую развязку.

Because без always governs the genitive in Russian.

• мотив = motive
• без мотива = without a motive

So this is not specific to this sentence; it is just the normal pattern:

• без сахара = without sugar
• без проблем = without problems
• без мотива = without a motive

Вряд ли means something like hardly, unlikely, or probably not.

In this sentence, it weakens the statement a little compared with a direct negation.

• не поверила бы = would not have believed
• вряд ли поверила бы = would hardly have believed / probably would not have believed

So вряд ли often adds a nuance of doubt or low probability rather than blunt refusal.

Russian forms the conditional with:

• a past-tense form
• plus the particle бы

So:

• поверила = believed
• поверила бы = would have believed / would believe

This is the standard Russian way to make a conditional. There is no special separate verb form like English would believe.

In this sentence, поверила бы expresses a hypothetical result: if there were no motive, the investigator would hardly have believed such an ending.

Because the subject is следовательница, which is a feminine noun.

In the Russian past tense, the verb agrees with the subject in gender and number:

• masculine: поверил
• feminine: поверила
• neuter: поверило
• plural: поверили

So следовательница ... поверила is the correct agreement.

Следовательница explicitly means a female investigator.

• следователь = investigator
• следовательница = female investigator

A learner should know that profession nouns in Russian can be a little stylistically sensitive. In some official or neutral contexts, Russian may still use the masculine profession noun even for a woman. But следовательница clearly marks the person as female and sounds natural in many contexts.

The verb верить / поверить can take в + accusative when it means to believe in something, or to accept something as real/plausible.

So here:

• поверить во что? = to believe in what?
• в такую развязку = in such an ending / outcome / resolution

This is just the verb’s normal pattern, like:

• поверить в чудо = to believe in a miracle
• поверить в победу = to believe in victory

So even though в often makes learners think about motion vs location, here it is simply part of the verb construction поверить в + accusative.

Because такой must agree with развязку in gender, number, and case.

Here, развязка is:

• feminine
• singular
• accusative

So такой becomes такую.

Compare:

• nominative: такая развязка
• accusative: в такую развязку

Because поверить and верить are related but not identical in meaning.

• верить = to believe, to have faith, to believe over time
• поверить = to come to believe, to accept as true

In this sentence, the idea is not about a continuing state of belief. It is about a single mental step: whether she would have accepted that explanation/outcome.

So поверила бы means roughly: she would have come to believe / accepted it.

If you used верила бы, it would suggest more of an ongoing state: she would believe.

Literally, развязка can mean denouement, resolution, or ending, especially in a story or complicated situation.

In this sentence, it likely means something like:

• the final outcome
• the way events turned out
• the explanation at the end

So в такую развязку means she would hardly have believed in such an outcome or such a resolution of events.

Russian word order is flexible, and the beginning of the sentence often highlights the most important background condition.

By putting Без мотива first, the sentence emphasizes the key point:

Without a motive, the investigator would hardly have believed such an ending.

This fronting sounds natural because it frames the whole statement. English often does something similar: Without a motive, ...

No. Бы is fairly mobile in Russian, though not completely free.

In this sentence, поверила бы is very normal. But you may also see variations such as:

• Без мотива следовательница вряд ли бы поверила в такую развязку.
• Без мотива следовательница бы вряд ли поверила в такую развязку.

Some placements sound more natural than others, but the basic meaning stays the same. Learners should remember that бы often appears near the verb or after an early important word in the clause.

So поверила бы is standard and easy to understand, but it is not the only possible placement.