Learn Hardcore Greek: Όταν ένα παιδί πλησιάζει την τσουλήθρα τρέχοντας, ο φύλακας του λέει να προσέχει. - When a child approaches the slide running, the guard tells them to be careful.

Όταν means “when” in the sense of “whenever / every time that” and is used for time clauses.
In this sentence it introduces a general situation: Whenever a child approaches the slide running…

Αν means “if” and introduces a condition, not a time.
You would not normally use αν here, because the point is when this happens in time, not if this happens or not.

Why is it ένα παιδί and not just παιδί?

Ένα is the indefinite article (“a / one”) in the neuter singular.
Παιδί is a grammatically neuter noun, so the neuter article ένα is used.

ένα παιδί = “a child” (non‑specific)
bare παιδί is less common in this position and would sound a bit more like a label or generic term rather than “a (random) child”.

So Όταν ένα παιδί… is naturally translated “When a child… / Whenever a child…”.

Why is παιδί neuter and how does that affect the rest of the sentence?

In Greek, many words for children or young beings are neuter, including το παιδί (“the child”), regardless of the child’s biological sex.
Because παιδί is neuter, any article, adjective, or pronoun that refers to it must also be neuter in form.

Here you see:

ένα παιδί – neuter article + neuter noun
Later, the pronoun του refers back to this child, but του is the genitive singular clitic form that is the same for masculine and neuter, so you just learn it as “to him / to it / to the child” from context.

Why is it την τσουλήθρα and not η τσουλήθρα?

Η τσουλήθρα is the nominative (subject) form: “the slide”.
In the sentence, την τσουλήθρα is the direct object of πλησιάζει (“approaches”), so it must be in the accusative case.

Feminine singular definite article:

nominative: η τσουλήθρα (subject)
accusative: την τσουλήθρα (object)

So πλησιάζει την τσουλήθρα = “(he/she) approaches the slide.”

What exactly does πλησιάζει mean, and what tense/aspect is it?

Πλησιάζει is the 3rd person singular, present tense, active of πλησιάζω.
It generally means “approaches / comes closer (to)”.

In the present tense it can express:

what is happening now: “a child is approaching the slide”
or a general / repeated action: “whenever a child approaches the slide”

Here, with Όταν, it has that general, habitual meaning: Whenever a child approaches the slide…

What is τρέχοντας, and how is it formed?

Τρέχοντας is an adverbial participle (often called the “-οντας form”), from the verb τρέχω (“to run”).
It roughly corresponds to English “running” used adverbially:

πλησιάζει την τσουλήθρα τρέχοντας
= “(he/she) approaches the slide running” / “runs up to the slide”.

It is formed by taking the 1st person plural present of the verb (τρέχουμε) and replacing -ουμε with -οντας → τρέχοντας.
This participle describes how or in what manner the action of the main verb is done.

Could τρέχοντας go somewhere else in the sentence?

Yes, Greek word order is relatively flexible, especially for such participles.
You might also hear:

Όταν ένα παιδί τρέχοντας πλησιάζει την τσουλήθρα…
Όταν, τρέχοντας, ένα παιδί πλησιάζει την τσουλήθρα… (more marked/stylistic)

However, the most natural, neutral position is usually right after the main verb phrase, as in the original:
πλησιάζει την τσουλήθρα τρέχοντας.
Moving it too far away can sound a bit more literary or awkward in everyday speech.

What is the role of ο φύλακας in the sentence?

Ο φύλακας (“the guard”) is the subject of the second clause.
He is the one performing the action of λέει (“says / tells”).

So the structure is:

Όταν ένα παιδί πλησιάζει…, ο φύλακας του λέει να προσέχει.
= “When a child approaches…, the guard tells him to be careful.”

Ο is the masculine singular nominative article; φύλακας is a masculine noun meaning a guard, watchman, keeper, etc.

What does του mean here, and why is that form used?

Του here is a clitic pronoun in the genitive singular, functioning as an indirect object: “to him / to the child”.
So ο φύλακας του λέει… means “the guard says to him / tells him…”.

In modern Greek, indirect objects are very often expressed with:

a genitive clitic (like του, της, τους)
instead of a full phrase with σε (e.g. στο παιδί).

You could also say: Ο φύλακας λέει στο παιδί να προσέχει, which is more explicit but a bit longer.

Why is it του λέει να προσέχει and not something like τον λέει να προσέχει?

Του and τον are different cases and functions:

του = genitive singular clitic → typical for indirect object (“to him”)
τον = accusative singular clitic → typical for direct object (“him” as the thing affected)

The verb λέω in this meaning usually takes:

a direct object (what is said) and
an indirect object (to whom it is said).

Here, the direct object is the whole clause να προσέχει (what he is telling), and the person being told is του (to him).
So του λέει να προσέχει is the normal pattern: “(he) tells him to be careful.”

What does the construction λέει να προσέχει mean exactly? Is να προσέχει like an infinitive?

Modern Greek does not have an infinitive in normal use.
Instead, it uses να + subjunctive to express meanings that English often uses an infinitive for.

So:

λέει να προσέχει ≈ “(he) tells (him) to be careful”.

Here να προσέχει is a subjunctive clause functioning a bit like an infinitive+object in English.
The guard is not just “saying that he is careful”, he is instructing / telling him to be careful.

Why is it να προσέχει and not να προσέξει?

Both come from προσέχω (“to pay attention / be careful”), but they differ in aspect:

να προσέχει – imperfective aspect → ongoing / continuous / habitual
“to be (continuously) careful / to keep being careful”
να προσέξει – perfective aspect → single, complete action
“to be careful (this time) / to take care (once / at that moment)”

In this general, habitual context (whenever a child approaches the slide), να προσέχει fits better, because the guard wants the child to be careful as a general, ongoing behavior, not just in a single moment.

How would the sentence change if we were talking about the past instead of a general present/habitual situation?

To put this into a past habitual or repeated past meaning, you would typically change both verbs to past imperfect:

Όταν ένα παιδί πλησίαζε την τσουλήθρα τρέχοντας, ο φύλακας του έλεγε να προσέχει.
= “Whenever a child approached the slide running, the guard would tell him to be careful.”

Here:

πλησίαζε (imperfect) → repeated or ongoing action in the past
έλεγε (imperfect) → repeated action in the past

The να προσέχει part can stay the same, because the instruction is still about being careful (ongoing behavior), even though it was given in the past.