Learn Farsi: اقدامات و فعالیت‌های وی تاثیرگذار بود. - His actions and activities were influential.

In this sentence, وی is connected to the preceding nouns via the Ezafe vowel. Just like you would say کتاب او (his or her book) in everyday Farsi, here we use فعالیت‌های وی to mean "his or her activities" in a more formal, written register. The pronoun وی itself doesn't change form to become possessive; the Ezafe handles that connection.

Since the subjects are plural (actions and activities), why is the verb بود (was) singular?

In Farsi, when the subject of a sentence is inanimate (not a living person or animal), the verb is typically conjugated in the third-person singular, even if the subject is plural. Since اقدامات (actions) and فعالیت‌ها (activities) are inanimate concepts, they take the singular verb بود instead of the plural بودند.

How is the word تاثیرگذار (influential) constructed?

It is a compound adjective made of two parts: تاثیر (effect or influence) and گذار (the present stem of گذاشتن, meaning to put or place). So literally, it means "effect-placing," which translates smoothly to "influential" in English.

Why does اقدامات end in ات while فعالیت‌های uses the standard plural ها?

اقدامات uses the Arabic plural suffix ات (-at), which is very common in formal Farsi for nouns of Arabic origin. is also an Arabic borrowing, and while it can sometimes take the suffix, it is very common and completely standard to pluralize it with the native Farsi suffix (-ha), as seen here. Formal sentences often naturally mix these two plural forms.