Εκείνος μιλάει αργά, οπότε τον καταλαβαίνω καλύτερα.

Yes, Greek very often drops subject pronouns, because the verb ending already shows the person and number.

So μιλάει already means he/she speaks.

Using Εκείνος adds emphasis or contrast. It can feel like:

• He speaks slowly...
• That guy speaks slowly...
• As for him, he speaks slowly...

So this sentence could also be:

Μιλάει αργά, οπότε τον καταλαβαίνω καλύτερα.

That would sound more neutral, with less emphasis on he.

Εκείνος literally means that one or that person, and it can also mean he depending on context.

In this sentence, it works as a masculine singular subject pronoun:

• εκείνος = he

Greek has several demonstrative/pronominal forms like:

• αυτός = this one / he
• εκείνος = that one / he

Very often, εκείνος can sound a bit more emphatic, more distant, or more contrastive than αυτός.

Μιλάει is the 3rd person singular form of the verb μιλάω / μιλώ = to speak.

Here is the idea:

• μιλάω / μιλώ = I speak
• μιλάς = you speak
• μιλάει / μιλά = he/she speaks

So εκείνος μιλάει = he speaks

Also, in modern Greek, both μιλάει and μιλά are common for he/she speaks.
So you may also see:

• Εκείνος μιλά αργά...

Both are correct.

Αργά is an adverb, meaning slowly.

It describes how he speaks:

• μιλάει αργά = he speaks slowly

Compare:

• αργός = slow (masculine adjective)
• αργή = slow (feminine adjective)
• αργό = slow (neuter adjective)
• αργά = slowly (adverb)

So:

• αργός άνθρωπος = a slow person
• μιλάει αργά = he speaks slowly

Yes, here οπότε means so, therefore, or as a result.

It connects the two ideas:

• He speaks slowly,
• so I understand him better.

So in this sentence, οπότε introduces the result or consequence.

A few rough English equivalents:

• so
• therefore
• which means
• as a result

In everyday Greek, οπότε is very common in speech.

Τον is the weak object pronoun meaning him.

In Greek, object pronouns usually come in short unstressed forms:

• με = me
• σε = you
• τον = him
• την = her
• το = it

So:

• τον καταλαβαίνω = I understand him

Αυτόν is a stronger/emphatic form, more like:

• him
• that one
• that man

You would use αυτόν for emphasis or contrast, but normally Greek prefers the weak clitic τον before the verb.

Because Greek weak object pronouns normally go before the verb.

So Greek says:

• τον καταλαβαίνω = literally him I-understand

This is a very normal Greek pattern.

Examples:

• Τον βλέπω. = I see him.
• Την ξέρω. = I know her.
• Το θέλω. = I want it.

In English, object pronouns usually come after the verb, but in Greek these short forms usually come before it.

Yes, καταλαβαίνω means I understand.

It can be used for:

• understanding what someone says
• understanding an idea
• understanding a situation

In this sentence:

• τον καταλαβαίνω = I understand him

Literally, that usually means I can understand what he is saying, not necessarily I sympathize with him.

Both can mean better.

• καλύτερα = better
• πιο καλά = more well / better

In many situations, they are interchangeable.
But καλύτερα is the standard comparative form and is very common.

So:

• τον καταλαβαίνω καλύτερα
• τον καταλαβαίνω πιο καλά

Both are possible, though καλύτερα is often the more natural choice.

Yes. Here καλύτερα is an adverb, because it describes how well I understand him.

• καλά = well
• καλύτερα = better

So:

• τον καταλαβαίνω καλά = I understand him well
• τον καταλαβαίνω καλύτερα = I understand him better

Yes, Greek word order is more flexible than English, though not completely free.

The neutral order here is:

• Εκείνος μιλάει αργά, οπότε τον καταλαβαίνω καλύτερα.

But you may also hear variations depending on emphasis, such as:

• Εκείνος μιλάει αργά, οπότε καλύτερα τον καταλαβαίνω.
  This is less neutral and sounds marked.
• Μιλάει αργά, οπότε τον καταλαβαίνω καλύτερα.
  More neutral, with no emphatic subject pronoun.

So yes, word order can shift, but the given version is very natural.

Yes. The comma helps separate the first clause from the result clause.

• Εκείνος μιλάει αργά,
• οπότε τον καταλαβαίνω καλύτερα.

This is similar to English:

• He speaks slowly, so I understand him better.

In Greek, punctuation with connectors like οπότε often works similarly to English.

The verb μιλάει by itself could mean he speaks or she speaks, because Greek verbs do not mark gender.

But the pronouns make the gender clear here:

• Εκείνος = he / that man
• τον = him

So this sentence is definitely referring to a male person.

If it were she, you would expect:

• Εκείνη μιλάει αργά, οπότε την καταλαβαίνω καλύτερα.

Yes, the present tense here suggests a general or habitual situation.

• Εκείνος μιλάει αργά = He speaks slowly
• τον καταλαβαίνω καλύτερα = I understand him better

This sounds like a general fact, not just a one-time event.

So the sentence means something like:

• whenever he speaks, he tends to speak slowly, and as a result I understand him better.

If you wanted a one-time past event, the Greek would be different.