Η άφιξη φαίνεται στην οθόνη, αλλά η ώρα αλλάζει συνέχεια.

Because both nouns are the subjects of their clauses, so they are in the nominative case.

• η άφιξη = the arrival
• η ώρα = the time

In Greek, the feminine singular nominative definite article is η.

If these nouns were direct objects, you would normally see την instead:

• Βλέπω την άφιξη. = I see the arrival.

Here, though:

• Η άφιξη φαίνεται... = The arrival appears / is shown...
• ...η ώρα αλλάζει... = ...the time changes...

So η is correct.

φαίνεται comes from φαίνομαι, which usually means:

• to appear
• to be visible
• to seem

In this sentence, it most naturally means something like:

• appears
• is shown
• is visible

So Η άφιξη φαίνεται στην οθόνη means the arrival is visible or displayed on the screen.

A useful point: φαίνεται can mean different things depending on context.

• Φαίνεται κουρασμένος. = He seems tired.
• Το βουνό φαίνεται από εδώ. = The mountain can be seen from here.

Here it is the visible/displayed meaning.

Because στην is the common contracted form of:

• σε + την = στην

So:

• στην οθόνη = on the screen / to the screen / in the screen, depending on context

With φαίνεται, στην οθόνη means on the screen.

Also note that after σε, Greek uses the accusative:

• nominative: η οθόνη
• accusative: την οθόνη

So the full form would be σε την οθόνη, but in normal Greek this becomes στην οθόνη.

It is changing case, but for this noun the nominative and accusative singular happen to look the same.

For οθόνη:

• nominative: η οθόνη
• accusative: την οθόνη

So the article changes clearly, but the noun itself does not look different in writing.

This is common with many feminine nouns in -η.

ώρα can mean both:

• hour
• time

The exact meaning depends on context.

In this sentence, η ώρα αλλάζει συνέχεια does not mean the hour changes constantly in the literal sense of 1 o’clock, 2 o’clock, etc. It means the time display keeps changing, so English usually says the time.

Other examples:

• Τι ώρα είναι; = What time is it?
• Μία ώρα = one hour

So ώρα is broader than just hour.

αλλάζει comes from αλλάζω = to change.

συνέχεια literally relates to continuity, but very often it is used adverbially to mean:

• constantly
• continuously
• all the time

So:

• η ώρα αλλάζει συνέχεια = the time keeps changing / the time changes constantly

This is a very common everyday use of συνέχεια.

Examples:

• Μιλάει συνέχεια. = He/She talks all the time.
• Βρέχει συνέχεια. = It keeps raining.

Because Greek often expresses that idea with the present tense plus an adverb like συνέχεια.

So English might say:

• keeps changing
• changes constantly
• is always changing

Greek can simply say:

• αλλάζει συνέχεια

The idea of repeated or ongoing change comes from:

1. the present tense (αλλάζει), and
2. the adverb συνέχεια

Greek does not need a separate verb equivalent to English keep here.

Because the subject is singular:

• η ώρα = the time

So the verb must also be singular:

• η ώρα αλλάζει = the time changes

If the subject were plural, the verb would be plural too:

• Οι ώρες αλλάζουν. = The times/hours change.

The same applies in the first clause:

• Η άφιξη φαίνεται
  Both subject and verb are singular.

Formally, it uses a mediopassive ending, and learners often notice that right away. But the most helpful thing is to learn φαίνομαι as its own verb meaning:

• appear
• seem
• be visible

So although it looks passive in form, you should not always translate it as a normal English passive.

For example:

• Φαίνεται ωραίο. = It seems nice.
• Το όνομα φαίνεται στην οθόνη. = The name appears/is shown on the screen.

In this sentence, is shown works in English, but Greek is simply using the normal verb φαίνομαι.

Because αλλά means but, and Greek normally uses a comma before it when it connects two clauses.

So the structure is:

• Η άφιξη φαίνεται στην οθόνη,
• αλλά η ώρα αλλάζει συνέχεια.

This is very similar to standard English punctuation before but when two full clauses are joined.

Yes. Greek word order is more flexible than English, although not random.

The given sentence is natural and neutral:

• Η άφιξη φαίνεται στην οθόνη, αλλά η ώρα αλλάζει συνέχεια.

But Greek could move things around for emphasis, for example:

• Στην οθόνη φαίνεται η άφιξη...
  This puts more focus on on the screen.

Even so, the original order is perfectly normal and probably the most straightforward for a learner.

Greek uses the definite article much more often than English.

So where English might say:

• arrival
• time

Greek often says:

• η άφιξη
• η ώρα

In this sentence, both nouns are understood as specific:

• the arrival being referred to
• the time being shown

That makes the article very natural. In general, learners should get used to Greek using articles in places where English may omit them.

The written accents show you where the stress goes:

• Η άφιξη → stress on ά: A-fi-xi
• φαίνεται → stress on φαί: FE-ne-te
• στην οθόνη → stress on νό: o-THO-ni
• αλλά → stress on λά
• η ώρα → stress on ώ
• αλλάζει → stress on λά
• συνέχεια → stress on χει

Paying attention to these accents is important, because stress is a real part of the word in Greek.