دوستی که آنجا بود، مسیر را خیلی خوب به ما نشان داد.

Here دوستی means a friend.

The final -ی is an indefinite marker, so:

• دوست = friend
• دوستی = a friend / some friend

A tricky point: دوستی can also mean friendship in other contexts. Persian writing often leaves this kind of ambiguity to context. In this sentence, it clearly means a friend, because that person is the one who was there and showed the route.

که introduces a relative clause. Here it means who, that, or which, depending on context.

So:

• دوستی که آنجا بود = a friend who was there

Unlike English, Persian usually uses the same word که for people and things:

• مردی که... = the/a man who...
• کتابی که... = the/a book that...

Because relative clauses with که attach directly to the noun.

Compare:

• دوستِ خوب = a good friend
  
  • here you need ezafe
• دوستی که آنجا بود = a friend who was there
  
  • here you use که, not ezafe

So this is normal Persian structure.

It literally means was there.

• آنجا = there
• بود = was

Persian often puts location words before the verb, so آنجا بود is the normal order for was there.

را marks the direct object.

So:

• مسیر را = the route / the path as the thing being shown

It usually tells you that the noun is specific or known in the context. It often does not get translated as a separate word in English.

In spoken Persian, را is usually pronounced رو.

Because in Persian, adjectives often also work as adverbs.

So:

• خوب can mean good
• with a verb, خوب often means well

Here خیلی خوب modifies نشان داد, so it means showed very well or more naturally showed very clearly / very well.

Because Persian uses به for the recipient of actions like giving, showing, saying, and similar verbs.

So:

• به ما = to us

English often drops to in sentences like showed us the way, but Persian usually keeps it:

• مسیر را به ما نشان داد = literally showed the route to us

It is a compound verb: نشان دادن, meaning to show.

• نشان = sign / indication / showing
• داد = gave

But you should learn نشان دادن as a single unit meaning to show, not as a word-for-word phrase like give a sign.

This is very common in Persian. Many verbs are made from:

• a noun/adjective part
• plus a light verb such as کردن, دادن, زدن, شدن

Both are in the simple past.

• بود = was
• داد = gave; in نشان داد, it means showed

So the sentence describes a past situation:

• the friend was there
• and then showed the route

Not completely, but the sentence follows a very normal Persian pattern: verb at the end.

Main parts:

• دوستی که آنجا بود = subject
• مسیر را = direct object
• خیلی خوب = adverb phrase
• به ما = indirect object / recipient
• نشان داد = verb

A very literal order is:

• A friend who was there, the route very well to us showed

Persian allows some movement of these parts, but the verb usually stays at or near the end in neutral style.

A natural spoken version would be:

دوستی که اونجا بود، مسیر رو خیلی خوب بهمون نشون داد.

Common spoken changes:

• آنجا → اونجا
• را → رو
• به ما → بهمون
• نشان داد → نشون داد

So if you hear the spoken form, it is still the same sentence, just less formal.