Breakdown of Добавьте эту пачку к десяти пачкам, пожалуйста.
этот
this [f: эта, n: это]
десять
ten
пожалуйста
please
к
towards, to
Questions & Answers about Добавьте эту пачку к десяти пачкам, пожалуйста.
Why does the number 10 change to десяти here?
The preposition к (to) always requires the Dative case. Numbers from 5 to 20 decline just like feminine nouns ending in a soft sign, such as тетрадь. In the Dative case, the soft sign changes to -и, making it десяти.
Wait, shouldn't the noun after 'ten' be in the Genitive plural (пачек)?
That is only true when the number is in the Nominative or inanimate Accusative case. In all other 'oblique' cases, like the Dative case here, the noun stops taking the Genitive and instead simply matches the case of the number. Since the number is Dative (десяти), the noun must be Dative plural (пачкам).
Why is эту пачку in the Accusative case?
It is the direct object of the verb добавьте (add). The structure is to add something (Accusative) to something else (к + Dative). Since the word for pack (пачка) is a feminine noun, эта пачка changes to эту пачку.
Is добавьте perfective or imperfective?
It is the perfective verb добавить, used here in the formal or plural imperative (command) form. You use the perfective aspect here because you are asking someone to complete a single, specific action with a concrete result.