Στενοχωριέμαι όταν αργεί να απαντήσει η φίλη μου, γιατί νομίζω ότι κάτι έχει συμβεί.

It can cover all of those a bit, depending on context. In this sentence, στενοχωριέμαι suggests something like I get upset / I feel distressed / I worry.

It is a very common Greek verb for feeling emotionally troubled. Here, because the speaker is afraid that something may have happened, I get worried/upset is probably the best nuance.

Because στενοχωριέμαι is the form that means I feel upset / I get upset.

• στενοχωρώ = I upset someone
• στενοχωριέμαι = I get upset / I feel upset

So the ending -μαι here does not make it truly passive in English. It is one of those Greek verbs that has a middle/passive-looking form but is used with an active meaning in everyday speech.

Because Greek normally uses the definite article with possessive expressions:

• η φίλη μου = my friend
• literally: the friend of mine

This is completely normal Greek. English usually says my friend without the, but Greek usually wants the article.

This structure means she is late to answer, she is slow to answer, or she takes a long time to answer.

The pattern is:

• αργώ να + verb

So:

• αργεί να απαντήσει = she is late/slow to answer

This is a very common Greek structure.

Yes, να μου απαντήσει would also be perfectly natural if you want to make to me explicit.

Greek often leaves things out when they are obvious from context. Since the speaker is talking about their own friend not replying, it is easy to understand that the friend is not replying to the speaker.

So both are possible:

• αργεί να απαντήσει = she is slow to reply
• αργεί να μου απαντήσει = she is slow to reply to me

The second is a bit more explicit.

Because Greek does not use an infinitive like English does. Instead, it often uses:

• να + verb

After αργώ, Greek needs this kind of structure:

• αργεί να απαντήσει

So where English says to answer, Greek says να απαντήσει.

This is about aspect.

• να απαντήσει is the perfective subjunctive
• να απαντά would be the imperfective subjunctive

After αργεί να..., Greek usually prefers the perfective form when talking about the completion of a single action:

• αργεί να απαντήσει = she is taking a long time to give a reply

That is the natural choice here.

Yes. η φίλη μου is the subject of αργεί.

Greek word order is more flexible than English word order. So:

• όταν αργεί να απαντήσει η φίλη μου
• όταν η φίλη μου αργεί να απαντήσει

both work.

Putting the subject later can sound very natural and smooth in Greek.

Yes. You can say:

• Στενοχωριέμαι όταν η φίλη μου αργεί να απαντήσει...

This is also correct and natural. The original version just uses a different word order, which Greek allows much more freely than English.

Here γιατί means because.

Greek uses the same written word γιατί for:

• because
• why?

You understand which one it is from context.

In this sentence, it clearly means because:

• ..., γιατί νομίζω... = ..., because I think...

Yes, both are possible.

• νομίζω ότι...
• νομίζω πως...

Both mean I think that...

In modern Greek, ότι and πως are often interchangeable after verbs like νομίζω. ότι is slightly more neutral and standard in many contexts, but πως is also very common.

έχει συμβεί is the present perfect: something has happened.

That tense works well here because the speaker is thinking about a possible event in the past that still matters now. In other words:

• she has not replied
• therefore the speaker thinks maybe something has happened

If you said κάτι συνέβη, that would mean something happened, which is also possible in some contexts, but έχει συμβεί fits the idea of a past event with present relevance especially well.

It comes from συμβαίνω = to happen.

This verb is somewhat irregular in its stems:

• present: συμβαίνω
• perfective/aorist-related stem: συμβ-
• subjunctive here: να συμβεί
• perfect here: έχει συμβεί

So even though συμβεί looks different, it belongs to συμβαίνω.

As written, it most naturally sounds like a general or repeated situation:

• Στενοχωριέμαι όταν...
• I get upset when...

That is because the main verbs are in the present tense:

• στενοχωριέμαι
• αργεί
• νομίζω

So it feels like: Whenever my friend takes long to reply, I get worried...

If you wanted one specific past occasion, you would normally change the tenses, for example:

• Στενοχωρήθηκα όταν άργησε να απαντήσει η φίλη μου...

Yes, depending on the nuance you want.

For example:

• ανησυχώ = I worry / I am worried
• στενοχωριέμαι = I get upset / distressed
• αγχώνομαι = I get anxious / stressed

The original sentence is natural because it mixes emotional upset with worry. But if you wanted a more purely worry-focused sentence, ανησυχώ would also fit very well.