Learn Farsi: تغذیه‌ی خوب برای سلامتی خیلی مفید است. - Good nutrition is very beneficial for health.

The word تغذیه ends in a "silent heh" (ه) which is pronounced as an "e" vowel. When we link it to the adjective خوب (good) using the Ezâfe connector, we have to insert a "ye" sound so the vowels don't crash into each other. In modern Farsi writing, this is usually shown by adding a distinct ی, though you might sometimes see older texts use a little symbol over the final letter instead (تغذیۀ).

I know سلامت (salâmat) means "health." What does the extra ی at the end of سلامتی (salâmati) do here?

Both سلامت and سلامتی mean "health" and are often used interchangeably! Adding the suffix ـی (i) to the end of a word is a very common way in Farsi to create abstract nouns (similar to "-ness" or "-ity" in English). For general health and well-being, everyday speakers use سلامتی all the time.

Could I just use خوب (good) again instead of مفید (mofid)?

You definitely could, and it would be perfectly understood. However, مفید specifically means "beneficial" or "useful." Using it here makes your Farsi sound more natural and precise, just like choosing "beneficial for health" over "good for health" does in English.

Does the preposition برای (barâye) always translate exactly to "for" in sentences like this?

In the context of benefit or purpose—like being useful, good, or bad something—yes! maps perfectly to English here. Just remember that the word itself has a built-in , so it attaches smoothly to the next word without needing any extra sounds: .