Kuagana na marafiki wetu ni jambo gumu, lakini tutarudi kuwaona hivi karibuni.

Kuagana is the infinitive form (ku- + verb), and in Swahili the infinitive is often used like an English gerund (-ing) or abstract noun.
So Kuagana na marafiki wetu literally means To say goodbye to our friends, functioning as the subject of the sentence (like Saying goodbye to our friends).

• kuaga = to say goodbye (can be more one-directional: you say goodbye to someone)
• kuagana = to say goodbye to each other / to part ways (more mutual/reciprocal)

The -an- element often adds a reciprocal idea in Swahili verbs (doing the action “to/with each other”).

na can mean and, but it also commonly means with / to / by means of, depending on context.
In kuagana na marafiki wetu, na marks the people involved: say goodbye to/with our friends.

wetu means our, and it comes after the noun.

• marafiki = friends
• wetu = our

So the order is noun + possessive: marafiki wetu = our friends.

Possessives agree with the noun class. marafiki (plural of rafiki) is in a noun class that uses wa- agreement for people (often called class 2).
So:

• rafiki wangu = my friend
• marafiki wetu = our friends

For many non-people nouns you might see yetu, langu, zangu, etc., depending on the noun class.

ni is the common linking word for is/are in the present tense.
So:

• (Kuagana na marafiki wetu) ni jambo gumu = Saying goodbye to our friends is a difficult thing.

Swahili often uses ni to connect a subject to a noun phrase.

Swahili has no direct equivalents of a/an/the. Context handles definiteness.
jambo gumu is simply difficult thing / a difficult matter. If you need to specify, Swahili uses other tools (like demonstratives), not articles.

Yes, adjectives often show noun-class agreement.
Here the noun is jambo (class 5), and many adjectives in this class use a form like gumu (not mgumu).
So:

• jambo gumu = a difficult matter
• mambo magumu = difficult matters (plural of jambo is mambo, class 6, and the adjective becomes magumu)

tutarudi breaks down like this:

• tu- = we (subject marker)
• -ta- = will (future tense marker)
• -rudi = return / come back

So tutarudi = we will return.

kuwaona is:

• ku- = to (infinitive)
• -wa- = them (object marker for “they/them”)
• -ona = see

So kuwaona literally means to-see-them.

Yes, but the meaning changes slightly:

• tutarudi kuwaona = we will return to see them (explicit “them”)
• tutarudi kuona = we will return to see (more general—see what? see whom?)

If you want “them,” the -wa- object marker makes it clear.

hivi karibuni is a common phrase meaning soon / in the near future.

• karibuni relates to near/close
• hivi here helps form the fixed time expression (think “this soon / this near,” but you learn it as a set phrase: hivi karibuni).

You can often just memorize hivi karibuni as soon.