Learn Farsi: ما آن زن را می‌شناسیم.

ra is the definite direct object marker. In this sentence, the action of knowing is directed at that woman (an zan). Because that woman is a specific, known person (pointed out by the word an, meaning 'that'), Farsi requires you to put ra right after it to show she is the direct object receiving the action.

Does ra always come immediately after the noun it marks?

It comes at the very end of the entire object phrase. Here, the complete object is the phrase an zan (that woman). Therefore, ra goes at the end of that whole group of words, not directly attached to just the noun.

I have seen a different Farsi verb used for 'to know'. Why is mi-shenasim used here?

Farsi uses two different verbs for 'to know'. The verb here comes from shenakhtan, which means to be familiar or acquainted with a person, place, or thing. If you want to say you know a fact or a piece of information, you would use a different verb (daneshtan). Since we are talking about knowing a person, mi-shenasim is the correct choice.

What does the mi- prefix at the beginning of mi-shenasim do?

The mi- prefix is used to build present tense verbs in Farsi. When added to the present stem of the verb and the ending -im (which means 'we'), it creates the present tense meaning 'we know'.