Learn Farsi: عهد معنوی آنها، حرف نزدن با هیچ‌کس است. - Their spiritual vow is not speaking with anyone.

The infinitive حرف زدن means "to speak" or "speaking." To make an infinitive negative in Farsi, you simply add the negative prefix نـ (na-) directly to the verb stem, creating حرف نزدن (not speaking). In this sentence, it acts as a verbal noun functioning as the subject complement: "Their vow is [the act of not speaking]."

Why do we use هیچ‌کس (nobody) instead of "anyone" with the verbal noun حرف نزدن?

Farsi requires negative concord (double negatives). When an action is negative, words like "anyone" must take their negative forms like هیچ‌کس (nobody). This rule applies even when the "action" is a negative verbal noun (infinitive) like حرف نزدن, rather than a conjugated verb. So, literally, it translates to "not speaking with nobody."

Could we split the compound infinitive and say حرف با هیچ‌کس نزدن?

Yes, it is very common in Farsi to place prepositional phrases between the non-verbal and verbal halves of a compound verb. You could naturally say عهد معنوی آنها، حرف با هیچ‌کس نزدن است. Keeping حرف نزدن together as a single block, as done here, is also perfectly correct and emphasizes "not-speaking" as a single fixed concept.

How is the Ezafe chain structured in عهد معنوی آنها?

It chains a noun, an adjective, and a possessor. عهد (vow) takes an Ezafe to connect to the adjective معنوی (spiritual). Because معنوی ends in a vowel (ی), it takes a "ye" Ezafe to connect to the pronoun (them/their), resulting in "their spiritual vow."