Αν πονάει το πόδι σου, καλύτερα να μην τρέχεις σήμερα.

Because the grammatical subject is το πόδι σου — your leg/foot.

Greek often expresses this idea as:

Το πόδι μου/σου/του πονάει
literally: my/your/his leg hurts

So the verb agrees with το πόδι, which is singular and third person:

• το πόδι πονάει
• not το πόδι πονάς

English focuses more on you in sentences like your leg hurts, but Greek makes the leg/foot the subject.

It can mean either, depending on context.

In everyday Modern Greek, πόδι is often used for the whole lower limb, so it may be translated as:

• leg
• foot

In this sentence, either could fit depending on the situation. If someone has pain somewhere in that general area, Greek may still use πόδι.

So this is a very normal word to learn as a somewhat broader term than the English foot.

Because in Greek, unstressed possessive pronouns normally come after the noun:

• το βιβλίο μου = my book
• η φίλη σου = your friend
• το πόδι σου = your leg/foot

This is the standard pattern. English puts the possessive before the noun, but Greek usually puts it after.

So you should think of σου here as the normal way to say your.

Αν means if and introduces a condition.

So the sentence begins with a normal if-clause:

• Αν πονάει το πόδι σου = If your leg/foot hurts

This is a common, everyday structure in Greek for real or possible situations.

Because Greek commonly uses the indicative after αν in ordinary real/open conditions.

So:

• Αν βρέχει, μένουμε σπίτι. = If it’s raining, we stay home.
• Αν πονάει το πόδι σου... = If your leg/foot hurts...

A learner might expect να or a special subjunctive form after if, but that is not how this kind of Greek condition works. Here, αν + indicative is exactly what you expect.

Καλύτερα literally means better, but in this kind of sentence it means something like:

• it’s better to...
• you’d better...

So καλύτερα να μην τρέχεις means:

• it’s better not to run
• you’d better not run

Greek often leaves out an explicit it is here. The idea είναι καλύτερα is understood.

Because Greek often uses the pattern:

καλύτερα να + verb

to give advice or make a recommendation.

Examples:

• Καλύτερα να φύγουμε. = We’d better leave.
• Καλύτερα να περιμένεις. = You’d better wait.
• Καλύτερα να μην τρέχεις. = You’d better not run.

So να is part of the normal structure after καλύτερα when a verb follows.

Because μη(ν) is the negative particle used with:

• να-clauses
• the subjunctive
• imperatives

By contrast, δεν is used to negate the indicative.

So:

• τρέχεις = you run / you are running
• δεν τρέχεις = you are not running
• να τρέχεις = to run / that you run
• να μην τρέχεις = not to run / that you not run

Since the sentence has να, the correct negative is μην.

This is about aspect.

After να, Greek can use:

• present form: να τρέχεις
• aorist form: να τρέξεις

In this sentence, να μην τρέχεις suggests avoiding the activity of running in a general or ongoing sense — basically don’t be running / don’t go running today.

If you said να μην τρέξεις, that would sound more like avoiding a single complete action: don’t run as one event.

Both can be possible in some contexts, but να μην τρέχεις σήμερα feels very natural for advice about today’s activity.

You can hear both.

For the verb πονάω / πονώ, common present forms include:

• πονάει
• πονά

So both of these are normal:

• Το πόδι μου πονάει.
• Το πόδι μου πονά.

The version with πονάει is extremely common in everyday speech, so the sentence you have is perfectly natural.

Because Greek word order is fairly flexible, and putting σήμερα at the end is a natural, neutral choice.

This sentence could also be rearranged, for example:

• Καλύτερα να μην τρέχεις σήμερα.
• Σήμερα καλύτερα να μην τρέχεις.

The version you have sounds very normal. Final σήμερα simply adds the time reference in an unobtrusive way: today.

Yes. Greek could use a more direct negative command, for example:

• Μην τρέχεις σήμερα. = Don’t run today.

But καλύτερα να μην τρέχεις σήμερα sounds softer and more like advice:

• you’d better not run today
• it would be better not to run today

So the sentence is not a harsh order; it is a recommendation.