Après le sport, j'ai mal aux jambes et au dos.

In French, physical pain is expressed with the verb avoir (to have), not être (to be).

• J'ai mal literally = I have pain.
• Je suis mal means something more like I feel bad / I’m unwell / I’m in a bad situation, not that a specific body part hurts.

For body parts, French uses the pattern:
avoir mal à + definite article + body part

• J'ai mal à la tête. – My head hurts.
• J'ai mal aux jambes. – My legs hurt.

Word-for-word:

• j'ai – I have
• mal – pain / ache
• aux – to the (for a plural noun; = à + les)
• jambes – legs
• et – and
• au – to the (for a masculine singular noun; = à + le)
• dos – back

So literally: “I have pain in the legs and in the back.”
Natural English: “My legs and back hurt.”

Because jambes is plural and dos is singular masculine:

• à + les = aux → used before plural nouns
  
  • aux jambes (les jambes) – the legs
• à + le = au → used before masculine singular nouns
  
  • au dos (le dos) – the back

If it were a feminine singular noun, you would have:

• à + la = à la
  
  • J'ai mal à la tête. – My head hurts.
• à + l' = à l' (before a vowel sound)
  
  • J'ai mal à l'estomac. – My stomach hurts.

In this structure, the pattern is fixed:

avoir mal à + definite article + body part

The à + article is obligatory; you cannot say:

• ✗ J'ai mal les jambes.
• ✗ J'ai mal mes jambes.

Correct forms are:

• J'ai mal aux jambes. – My legs hurt.
• J'ai mal au dos. – My back hurts.

French usually uses the definite article (le, la, les) for body parts with this expression, not possessives (mes, ton, etc.) because the owner is clear from the subject (je).

Yes, that’s correct and natural. It uses a slightly different structure:

• Mes jambes / mon dos me font mal. – Literally: My legs / my back are causing me pain.

Both sentences are fine:

• Après le sport, j'ai mal aux jambes et au dos.
• Après le sport, mes jambes et mon dos me font mal.

The first is a bit more formulaic and is often what beginners learn first.

In French, you normally need an article before sport.

• Après le sport = after (the) sport / after sports / after my workout
  
  • le is the usual article when you’re talking about an activity in a general or routine sense:
    
    • J'aime le sport. – I like sports.
    • Après le sport, je prends une douche. – After sports / my workout, I shower.

Après du sport is possible but has a slightly different nuance:

• Après avoir fait du sport – after doing some sport / after exercising

You cannot drop the article:

• ✗ Après sport – not correct in standard French.

Because:

• les jambes – legs (you have two legs) → plural
• le dos – back (you only have one back) → singular

So:

• aux jambes = to the legs
• au dos = to the back

This is just reflecting the normal number of those body parts.

You can say either:

• Après le sport, j'ai mal aux jambes et au dos.
• Après le sport, j'ai mal au dos et aux jambes.

Both are grammatically correct and natural. There’s no strict rule here; it’s just style and what sounds more natural to the speaker.

In j'ai mal aux jambes, mal is a noun meaning pain / ache.

• avoir mal (à) = to have pain (in)
  
  • J'ai mal au dos. – I have back pain / my back hurts.

As an adjective, mal would mean bad (opposite of bien), for example:

• C'est mal. – That’s bad.

But in this expression, think of mal as “pain”.

Yes. The pattern avoir mal à + definite article + body part is very productive:

• J'ai mal à la tête. – My head hurts.
• J'ai mal au ventre. – My stomach/belly hurts.
• J'ai mal au genou. – My knee hurts.
• J'ai mal aux yeux. – My eyes hurt.
• J'ai mal aux oreilles. – My ears hurt.

You could say, for example:

• Après le sport, j'ai vraiment mal aux jambes.
  or
• Après avoir fait du sport, j'ai très mal aux jambes.

vraiment / très both intensify the pain (really / very).

No. J'ai mal is in the present tense: it describes how you feel now.

The full idea is:

• Après le sport, j'ai mal aux jambes et au dos.
  = Whenever / when I finish sports, I then have pain in my legs and back.

If you wanted to talk about a specific past time, you’d change the verb or add context:

• Hier, après le sport, j'avais mal aux jambes et au dos. – Yesterday, after sports, my legs and back hurt.

Pronunciation:

• aux in aux jambes → sounds like [o], same as au.
  
  • There is a liaison: mal‿aux jambes → you hear a z sound linking mal and aux: [maloz‿ʒɑ̃b].
• au in au dos → also [o], and usually no liaison here: [o do].

So aux and au are pronounced the same; they differ only in grammar (plural vs masculine singular).