Όταν ανέβηκα τη σκάλα τρέχοντας, χρειάστηκα λίγη ώρα για να πάρω ανάσα.

Here Όταν means when in the sense of a specific time in the past.

Because the verbs that follow are past, completed actions — ανέβηκα and χρειάστηκα — the sentence refers to one particular occasion:

When I ran up the stairs, I needed a little while to catch my breath.

In other contexts, όταν can also mean whenever, especially with present-tense or repeated actions.

Ανέβηκα is the aorist of ανεβαίνω, so it presents the action as a single completed event:

• ανέβηκα = I went up / I climbed / I ran up
• ανέβαινα = I was going up / I used to go up

In this sentence, the speaker is talking about one completed action, so the aorist is the natural choice.

Τη σκάλα is in the accusative, because it is the object of ανέβηκα.

In Greek, ανεβαίνω can work like go up / climb + thing climbed:

• ανεβαίνω τη σκάλα = go up the stairs/staircase
• ανεβαίνω το βουνό = climb the mountain

So Greek structures this more directly than English sometimes does.

Also, σκάλα can mean:

• staircase / flight of stairs
• ladder, depending on context

The feminine accusative article is often written as τη(ν) because the final -ν is sometimes kept and sometimes dropped.

Before σ, it is normally dropped, so:

• τη σκάλα is the usual form

The -ν is normally kept before vowels and certain consonants, for example:

• την ώρα
• την πόρτα

So τη σκάλα is completely normal and standard here.

Τρέχοντας means running, while running, or by running.

It is the adverbial participle (sometimes called the Greek gerund) of τρέχω. It describes how the action happened:

• ανέβηκα τη σκάλα τρέχοντας
  = I went up the stairs running = more naturally, I ran up the stairs

A useful point: this form is invariable and usually has the same subject as the main verb. So the person running is the same person who went up the stairs.

Greek often expresses this idea with a main verb plus an adverbial participle:

• ανέβηκα ... τρέχοντας = I went up ... running

That is a very natural Greek way to show both:

• the main action: going up
• the manner: running

So even though English often prefers ran up, the Greek structure is perfectly idiomatic.

Yes, it comes from χρειάζομαι, which looks like a middle/passive-form verb, but its meaning is active:

• χρειάζομαι = I need
• χρειάστηκα = I needed

So χρειάστηκα does not mean I was needed here. It means I needed.

The tense is again aorist, because the speaker is referring to a single completed situation in the past: after running upstairs, they needed some time.

If you used χρειαζόμουν, that would sound more ongoing or descriptive:

• I was needing / I needed for a while

Λίγη ώρα means a little while or a short time.

It is λίγη because ώρα is a feminine noun, so the adjective must agree with it:

• λίγος = masculine
• λίγη = feminine
• λίγο = neuter

So:

• λίγη ώρα = correct
• λίγο ώρα = incorrect

Also, ώρα literally means hour, but very often it simply means time / while in expressions like this.

Για να means to or in order to.

It introduces the purpose or goal:

• χρειάστηκα λίγη ώρα για να πάρω ανάσα
• I needed a little while to catch my breath

Literally, it is something like for to, but in natural English you usually just translate it as to or in order to.

After να, Greek uses the subjunctive, and Greek also chooses between aorist aspect and present aspect.

Here:

• να πάρω = aorist subjunctive
• να παίρνω = present subjunctive

Να πάρω is used because catching one’s breath is seen as a single whole event.

So:

• να πάρω ανάσα = to catch my breath

If you said να παίρνω ανάσα, it would sound more like an ongoing or repeated action, which does not fit as well here.

Παίρνω ανάσα is an idiomatic expression meaning:

• catch my breath
• take a breath

Here ανάσα means breath.

There is no article because this is a common general expression. Greek often leaves out the article in fixed, idiomatic phrases like this.

You may also hear:

• παίρνω μια ανάσα = I take a breath / I take a quick breather

But in this sentence, να πάρω ανάσα is very natural.

Greek word order is fairly flexible, but the given order is very natural.

The sentence starts with the Όταν clause to set the time frame first:

• Όταν ανέβηκα τη σκάλα τρέχοντας, ...

Then comes the main clause:

• χρειάστηκα λίγη ώρα για να πάρω ανάσα

This is a common pattern in both Greek and English: first give the background event, then give the main result.

The comma is also normal because the sentence begins with a subordinate time clause.