Πετυχαίνω καλύτερο αποτέλεσμα όταν επιλέγω λίγα αλλά σημαντικά πράγματα και δεν προσπαθώ να κάνω τα πάντα μαζί.

Here πετυχαίνω means achieve, get, or obtain a result.

It is a very common verb with a few related meanings, such as:

• succeed
• manage to achieve
• hit / reach a target

In this sentence, Πετυχαίνω καλύτερο αποτέλεσμα means that the speaker gets a better result.

Greek often leaves out subject pronouns when they are not needed.

The verb ending already shows the subject:

• πετυχαίνω = I achieve / I get
• επιλέγω = I choose
• προσπαθώ = I try
• κάνω = I do / I make

So adding εγώ would usually only be for emphasis or contrast.

Because the sentence expresses a general truth, habit, or usual pattern, not just one specific moment.

So Greek uses the present tense much like English does in sentences such as:

• I get better results when I focus
• I choose a few important things
• I do not try to do everything at once

This is a very normal use of the Greek present.

Όταν means when or whenever, depending on context.

In this sentence, because the verbs are in the present and the meaning is general, όταν is best understood as whenever:

• όταν επιλέγω... = whenever I choose...

So it introduces the condition or situation in which the better result happens.

Because καλύτερο is an adjective, while καλύτερα is usually an adverb.

Here καλύτερο describes the noun αποτέλεσμα, so it must agree with it.

• αποτέλεσμα is neuter singular
• therefore the adjective is also neuter singular: καλύτερο

So:

• καλύτερο αποτέλεσμα = a better result
• καλύτερα would mean something like better in the sense of more well, not a noun modifier here

Because Greek often omits the article when speaking about something indefinite or non-specific.

So καλύτερο αποτέλεσμα means a better result, not the better result.

If the speaker meant a specific result, Greek could use the article:

• το καλύτερο αποτέλεσμα = the best / the better result, depending on context

In your sentence, the meaning is general, so no article is perfectly natural.

Because they agree with πράγματα.

• πράγματα is neuter plural
• λίγα = few, in the neuter plural form
• σημαντικά = important, also in the neuter plural form

Greek adjectives and adjective-like words must agree with the noun in:

• gender
• number
• case

So all three match:

• λίγα
• σημαντικά
• πράγματα

Αλλά means but.

It connects two ideas that contrast slightly:

• λίγα = few
• σημαντικά = important

So the phrase means something like:

• few, but important, things

It highlights that the number is small, but the value or importance is high.

Because they are neuter nouns, and in Greek the nominative and accusative forms are often identical for neuter nouns.

Here they are direct objects:

• πετυχαίνω καλύτερο αποτέλεσμα
• επιλέγω λίγα ... πράγματα
• κάνω τα πάντα

Even though they function as objects, their forms do not visibly change.

That is very normal in Greek neuter nouns.

Because δεν is the standard negation word used before finite verbs in Greek.

So:

• προσπαθώ = I try
• δεν προσπαθώ = I do not try

Its usual position is directly before the verb it negates.

Because after προσπαθώ, Greek normally uses a να-clause.

So the structure is:

• προσπαθώ να + verb

Examples:

• προσπαθώ να μάθω = I try to learn
• προσπαθώ να καταλάβω = I try to understand
• προσπαθώ να κάνω = I try to do

This is one of the most common Greek patterns. In English we often use to + verb, but Greek usually uses να + verb.

Τα πάντα is a very common expression meaning everything.

Literally, it comes from all things, but in real use it simply means everything.

You can also hear όλα in many contexts, but τα πάντα is a fixed and very natural expression.

So:

• κάνω τα πάντα = do everything

It can sometimes sound slightly more complete or emphatic than just όλα.

Here μαζί means together in the sense of all at once or at the same time.

So να κάνω τα πάντα μαζί means:

• to do everything together
• to do everything all at once
• to do everything at the same time

In this sentence, that is the idea: trying to handle everything simultaneously.

Greek word order is fairly flexible, although the version you have is natural and neutral.

This sentence begins with Πετυχαίνω καλύτερο αποτέλεσμα, which puts the result first. But Greek could also rearrange parts for emphasis, for example by starting with the όταν clause.

The current order sounds smooth and standard. A different order would not necessarily change the basic meaning, but it could shift the emphasis.