Όταν καταλαβαίνω το νόημα και τη στίξη, διαβάζω ελληνικά πιο άνετα.

It can mean either, depending on context. In this sentence, because both verbs are in the present tense, Όταν has a general/habitual sense:

• Whenever I understand the meaning and the punctuation, I read Greek more comfortably.

English often still uses when for this kind of repeated situation, so the translation may simply say when even though the sense is close to whenever.

Greek uses the present tense here to describe a regular pattern or general truth, not a single event happening right now.

So this sentence means something like:

• When/whenever I understand ..., I read ... more comfortably.

If you wanted a one-time future meaning, Greek would normally change the structure, for example:

• Όταν καταλάβω το νόημα..., θα διαβάσω...
• When I understand the meaning..., I will read...

So the present tense here signals a repeated or usual situation.

Because the sentence begins with a subordinate clause introduced by Όταν.

Structure:

• Όταν καταλαβαίνω το νόημα και τη στίξη = subordinate clause
• διαβάζω ελληνικά πιο άνετα = main clause

In Greek, when that subordinate clause comes first, it is normally separated from the main clause with a comma.

Greek uses the definite article very often, more often than English in many contexts.

Here, το νόημα and τη στίξη are the direct objects of καταλαβαίνω, and the article makes them sound natural and specific:

• το νόημα = the meaning
• τη στίξη = the punctuation

Even when English might be a little looser, Greek often prefers the article.

The full feminine accusative singular article is την, but the final -ν is often dropped before certain consonants.

Since στίξη begins with σ, the normal form here is:

• τη στίξη

You usually keep the -ν before:

• vowels
• κ, π, τ, γκ, μπ, ντ, ξ, ψ

For example:

• την πόρτα
• την ξέρω
• την ψάχνω

So τη στίξη is exactly what you would expect.

They are in the accusative case, because they are the direct objects of καταλαβαίνω.

• καταλαβαίνω το νόημα
• καταλαβαίνω τη στίξη

In other words, they are the things being understood.

Here ελληνικά means Greek as a language or Greek material/texts, and after verbs like read, learn, or speak, Greek often uses the language name without the article.

So:

• διαβάζω ελληνικά = I read Greek
• μαθαίνω ελληνικά = I learn Greek
• μιλάω ελληνικά = I speak Greek

You can use the article τα in other contexts, especially when talking about the language as a subject or topic:

• Τα ελληνικά είναι δύσκολα.
• Greek is difficult.

This is very common with language names in Greek. Ελληνικά is a neuter plural form used as a noun, just like:

• αγγλικά = English
• γαλλικά = French
• ισπανικά = Spanish

Historically, these come from adjectives, but in modern Greek they function as the normal names of languages.

So even though English says Greek as a singular word, Greek says ελληνικά, which is grammatically plural/neuter in form.

Because πιο άνετα is working as an adverb, not an adjective.

It modifies the verb διαβάζω:

• διαβάζω ... πιο άνετα = I read ... more comfortably

Compare:

• άνετος / άνετη / άνετο = comfortable, at ease
  used for a person or thing
• άνετα = comfortably
  used for how an action is done

So:

• Είμαι πιο άνετος. = I am more comfortable / more at ease.
• Διαβάζω πιο άνετα. = I read more comfortably.

Greek very often makes comparatives with πιο + adjective/adverb.

So:

• άνετα = comfortably
• πιο άνετα = more comfortably

This πιο construction is extremely common and natural in modern Greek.

The word order is natural, but Greek is more flexible than English.

You could also say:

• Διαβάζω ελληνικά πιο άνετα όταν καταλαβαίνω το νόημα και τη στίξη.

That version is also correct. The meaning stays basically the same, but the focus changes a little depending on what you place first.

The original version is very natural because it starts with the condition and then gives the result.

In modern Greek, capital letters normally keep the accent mark in ordinary writing.

So:

• όταν becomes Όταν
• not Οταν

The accent mark shows which syllable is stressed. That is why you also see accents on words like:

• νόημα
• στίξη
• ελληνικά
• άνετα

It is an important part of standard Greek spelling.