Breakdown of С чьей помощью вы оцениваете подлинность статуэтки?
вы
you (plural)
чей
whose (m, sister forms: чья, чьё)
с
with (also со)
помощь
the help
Questions & Answers about С чьей помощью вы оцениваете подлинность статуэтки?
What case is чьей in this sentence, and why?
Чьей is the feminine singular instrumental form of чей (whose). It has to match the gender, number, and case of the noun it modifies, which is помощью (help). Since помощью is in the instrumental case after the preposition с (with), the possessive pronoun must also be in the instrumental case.
How does the noun помощью get its ending?
The dictionary form is помощь (help), which is a feminine noun ending in a soft sign (-ь). When put into the instrumental case (required here by the preposition с), feminine nouns of this type simply add -ю after the soft sign, resulting in помощью.
Since подлинность is the direct object being appraised, why doesn't its ending change?
It actually is in the accusative case here, as expected for a direct object! However, подлинность (authenticity) is an inanimate feminine noun ending in a soft sign. For this specific group of nouns, the accusative form looks exactly the same as the nominative (dictionary) form.
Why is статуэтки in the genitive case?