Ποτέ δεν πετυχαίνω καλό αποτέλεσμα όταν διαβάζω βιαστικά αργά τη νύχτα.

This is a very common Greek pattern. Greek usually uses negative concord, which means a negative word like ποτέ is normally used together with the negative particle δεν.

So:

• ποτέ δεν πετυχαίνω = I never achieve / I never get
• literally, it is something like never not-I-achieve

For an English speaker, this can feel like a double negative, but in Greek it is the normal way to say it.

A useful contrast:

• Έχεις πάει ποτέ; = Have you ever gone?
• Δεν έχω πάει ποτέ. = I have never gone.

So ποτέ can mean ever in questions, but in negative statements it works with δεν to mean never.

Putting ποτέ first is very natural because it gives strong emphasis to the idea of never right away.

Greek word order is more flexible than English, but this order is especially common:

• Ποτέ δεν πετυχαίνω...

It sounds natural and clear. You could sometimes hear other orders in different contexts, but this beginning is the most straightforward way to frame the whole sentence as a general negative statement.

Greek usually does not need subject pronouns, because the verb ending already shows the person and number.

Here:

• πετυχαίνω = I achieve / I get
• διαβάζω = I read / I study

The ending -ω tells you it is 1st person singular, so I is already built into the verb.

You would add εγώ only if you wanted emphasis or contrast, for example:

• Εγώ ποτέ δεν πετυχαίνω... = I never get good results...
  maybe implying but someone else does

Because the sentence describes a general habit or repeated truth, not one specific event.

In English, we also use the present for this kind of idea:

• I never get good results when I study in a rush late at night.

Greek does the same thing here:

• πετυχαίνω = I get / I achieve
• διαβάζω = I read / I study

These present-tense forms show something that happens regularly or whenever that situation occurs.

Because those forms would point to a different kind of meaning, usually more single, complete, or future-oriented.

Here the speaker is talking about a repeated pattern:

• Whenever I study hurriedly late at night, I never get a good result.

So Greek uses the imperfective present:

• πετυχαίνω
• διαβάζω

Compare:

• Όταν διαβάζω βιαστικά... = when/whenever I study hurriedly...
  habitual, repeated
• Όταν διαβάσω... = when I study / when I have studied...
  usually one future occasion

So the forms in your sentence are chosen because the meaning is general and habitual.

Πετυχαίνω has a few related meanings, such as:

• succeed
• achieve
• manage to get
• sometimes even hit in other contexts

In this sentence, with καλό αποτέλεσμα, it means something like:

• achieve
• get
• obtain

So:

• πετυχαίνω καλό αποτέλεσμα = I get/achieve a good result

That is a very natural Greek way to express the idea.

Because καλό is an adjective describing the noun αποτέλεσμα.

• αποτέλεσμα is neuter singular
• so the adjective must also be neuter singular
• that gives καλό

So:

• καλό αποτέλεσμα = good result

By contrast, καλά is usually an adverb, meaning well, so it would not fit here.

Compare:

• καλό αποτέλεσμα = a good result
• διαβάζω καλά = I study/read well

Greek can use an indefinite article, but it often leaves it out where English would use a.

So:

• καλό αποτέλεσμα can naturally mean a good result

This is especially common in general statements like this one.

If you said:

• ένα καλό αποτέλεσμα

that is also possible, but it can sound a little more specific, concrete, or emphatic depending on context.

So the version without ένα is very normal and natural here.

Όταν means when, but in sentences like this it often has the sense of when(ever).

So:

• όταν διαβάζω = when I read/study or whenever I read/study

Because the whole sentence is about a repeated situation, όταν + present verb works perfectly.

This is one of the most useful things to notice:

• όταν + present/imperfective often describes something habitual or repeated
• όταν + aorist/subjunctive-type form often refers to a single future event

So here όταν διαβάζω fits the meaning whenever I study.

Βιαστικά is an adverb meaning:

• hurriedly
• in a rush
• hastily

It describes how the studying/reading is done.

So:

• διαβάζω βιαστικά = I study/read hurriedly

Its position after the verb is very natural in Greek. Greek adverbs are somewhat flexible, but this is a common neutral order.

You could move it for emphasis in some contexts, but:

• όταν διαβάζω βιαστικά

is the most natural everyday phrasing here.

This is a standard time expression.

• αργά = late
• τη νύχτα = literally the night, but here it means at night

Together:

• αργά τη νύχτα = late at night

A couple of useful grammar points are hidden inside this:

1. τη(ν) νύχτα is in the accusative, which Greek often uses for expressions of time.
2. τη is a common reduced form of την before a consonant in everyday language and writing.

So τη νύχτα is completely normal.

Yes. That is one of the first things English speakers notice.

Διαβάζω can mean:

• read
• study

The exact meaning depends on context.

In a sentence about getting good or bad results, English often chooses study:

• when I study in a rush late at night

But the Greek verb is still διαβάζω.

So this is not a mistake or a special use — it is just how Greek works.

Yes, Greek has some flexibility, but the original order is natural:

• διαβάζω βιαστικά αργά τη νύχτα

This gives:

• first, the manner: βιαστικά = hurriedly
• then, the time: αργά τη νύχτα = late at night

You may also hear other orders for emphasis, but the sentence as given sounds smooth and idiomatic. For a learner, it is a good model to copy.