Learn Farsi: با اینکه نویسنده کتاب را ویرایش کرد، هیچ‌کس یک نسخه هم نخرید. - Even though the writer edited the book, nobody bought even one copy.

Because bâ inke states a fact rather than a possibility or wish, it takes normal indicative tenses. Here, the writer actually did edit the book, so we use the simple past tense virâyesh kard instead of the subjunctive.

Why is the verb nakharid (didn't buy) negative if the sentence already says hich-kas (nobody)?

Farsi requires double negatives. Whenever you use a negative pronoun like hich-kas (nobody) or hich-chiz (nothing), the verb must also be made negative. Literally, this translates to "nobody didn't buy".

What does the word ham mean after yek noskhe? Doesn't it usually mean "also"?

Yes, ham usually means "also" or "too". However, in negative sentences, it often translates to "even". Here, yek noskhe ham nakharid means "didn't buy even one copy".

Why do we use the object marker ra after ketab but not after yek noskhe?

The word ra is only used for specific, definite direct objects. ketab ra means "the book" (a specific book), while yek noskhe means "one copy" (indefinite), so it doesn't take ra.