Breakdown of Что бы обе сестры ни попросили, я сделаю всё.
сестра
sister
я
I
что
what
сделать
to do (perfective) / to make (perfective)
Questions & Answers about Что бы обе сестры ни попросили, я сделаю всё.
Why is the verb попросили in the past tense if the English translation says "ask" and it refers to the future?
This is the core rule for this grammar structure. Whenever you use the [Question word] + бы ни construction to mean "-ever" (whatever, wherever, whoever), the verb must be in the past tense. Even though the sentence refers to a future situation, the particle бы acts like a conditional marker and forces the past tense form.
Why are the particles бы and ни separated by обе сестры?
In these concessive clauses, it is very common and natural to place the subject or other words between [Question word] + бы and ни + [Verb]. You are also allowed to group the particles together right before the verb (for example: Что бы ни попросили обе сестры...). Both word orders are perfectly correct.
Why is it обе сестры, and what case is сестры in here?
The noun сестра is feminine, so you must use the special feminine form for "both", which is обе (the masculine/neuter form is оба). Just like the numbers two, three, and four in the nominative, обе requires the following noun to be in the genitive singular case, which gives us сестры.
What would happen if I accidentally used не instead of ни before попросили?