Learn Hardcore Greek: Η συγκοινωνία σήμερα δεν είναι καλή, οπότε υπολογίζω ότι θα αργήσω λίγο. - Public transport today isn’t good, so I figure I’ll be a bit late.

Η συγκοινωνία is a common Greek way to refer to public transport / the transport system / transit (in general) in an area. It’s often used as a collective singular (like English public transport), even though it covers many services (buses, metro, etc.).
You can also hear the plural οι συγκοινωνίες when people focus on the individual services/routes (roughly the transport links / the buses and trains).

Why is it Η and not Το or Ο?

Because συγκοινωνία is a feminine noun in Greek, so it takes the feminine definite article η (singular nominative).

feminine: η συγκοινωνία
masculine would take ο, neuter would take το

How does the negation work in δεν είναι καλή?

δεν is the standard negation used with the indicative (normal statement) forms of verbs.
So δεν είναι καλή literally means is not good.
Structure: δεν + verb → δεν είναι (is not).

Why is it καλή (feminine) and not another form?

Adjectives in Greek agree with the noun they describe in gender, number, and case.
Since συγκοινωνία is feminine singular, the adjective must be feminine singular too: καλή (good).

What exactly does σήμερα do, and can it move around in the sentence?

σήμερα means today. It’s an adverb of time and is quite flexible in position. For example, all of these are common:

Η συγκοινωνία σήμερα δεν είναι καλή.
Σήμερα η συγκοινωνία δεν είναι καλή.
The meaning stays basically the same; the placement can slightly change emphasis.

What does οπότε mean here, and how is it different from άρα?

οπότε here means so / therefore / as a result and links the first clause to a consequence. It often feels slightly conversational.
άρα also means therefore, and can sound a bit more “logical/structured.”
In many everyday sentences they’re interchangeable, but οπότε is very common in speech.

Why is there a comma before οπότε?

Because οπότε introduces a second clause that’s a result/conclusion. Greek punctuation commonly uses a comma to separate these linked clauses, similar to English:
..., so I think ...

What does υπολογίζω mean here, and is it literally “calculate”?

Literally υπολογίζω can mean calculate, but very commonly it means estimate / figure / assume / reckon.
In this sentence it’s the natural way to say I’m expecting / I figure I’ll be a bit late.

Why do we need ότι after υπολογίζω?

ότι introduces a content clause (what you think/estimate). It’s like English that:

Υπολογίζω ότι θα αργήσω λίγο. = I reckon (that) I’ll be a bit late.
Greek usually keeps ότι more often than English keeps that, though it can sometimes be omitted in casual speech.

Why does ότι have an accent, and is it different from ό,τι?

Yes—this is important:

ότι (no comma) = that (conjunction)
ό,τι (with comma) = whatever / anything that
Here it’s ότι because it means that.

How does the future work in θα αργήσω?

Greek forms the future with θα + verb.
In θα αργήσω, the verb αργήσω is an aorist-based form used after θα to express a future event: I will be late / I’ll arrive late.

What does αργήσω mean—“be late” or “delay”?

It can mean both depending on context. Here, with θα and the implied meaning “arrive later than expected,” it’s best understood as:
I’ll be late / I’ll arrive late.
In other contexts, it can also mean to take a long time (e.g., It will take a while).

What does λίγο add at the end?

λίγο means a little / a bit. Placed at the end, it softens the statement:
θα αργήσω λίγο = I’ll be a bit late (not very late).