Moje noge su umorne posle treninga.

Because moje has to agree with noge.

• noga = leg, feminine singular
• noge = legs, feminine plural

The possessive word moj changes form to match the noun:

• moja noga = my leg
• moje noge = my legs

So moja would be correct with singular noga, but not with plural noge.

Here noge is nominative plural because it is the subject of the sentence: the thing that is tired.

So in Moje noge su umorne, noge = my legs as the subject.

A useful detail: the form noge can mean different things in different contexts, including genitive singular, but in this sentence the verb su shows clearly that it is plural subject, so it must be nominative plural.

Su is the 3rd person plural present tense of biti = to be.

Since the subject is plural (noge), Serbian uses:

• je for singular
• su for plural

Compare:

• Moja noga je umorna. = My leg is tired.
• Moje noge su umorne. = My legs are tired.

In Serbian, this verb is normally kept in sentences like this.

Because the adjective has to agree with noge.

Noge is:

• feminine
• plural
• nominative

So the adjective must also be:

• feminine
• plural
• nominative

That gives umorne.

Compare:

• umoran = masculine singular
• umorna = feminine singular
• umorno = neuter singular
• umorni / umorne = plural, depending on gender pattern

So:

• Noga je umorna. = The leg is tired.
• Noge su umorne. = The legs are tired.

Because posle requires the genitive case.

The base noun is:

• trening = training, workout, practice

After posle, it becomes:

• posle treninga = after training / after the workout

So this is a normal preposition + case pattern in Serbian:

• posle + genitive

It is a borrowed word, ultimately from English, but it is fully normal in Serbian.

It behaves like a regular masculine noun:

• trening = nominative
• treninga = genitive
• treningu = dative/locative
• etc.

So even though it may look familiar to an English speaker, it still follows Serbian grammar like other nouns.

Yes. Both are correct.

• posle treninga
• nakon treninga

Both mean after training.

The difference is mostly style:

• posle is very common and everyday
• nakon can sound a little more formal or written

Also, in Ijekavian varieties you will often see poslije instead of posle.

Yes, Serbian often uses another very common pattern:

• Noge su mi umorne posle treninga.

Here mi means to me, but this structure is very often used where English uses my.

So both are correct:

• Moje noge su umorne posle treninga.
• Noge su mi umorne posle treninga.

The second version is often more natural in everyday speech. The first one can sound a bit more explicit or emphatic: my legs.

No. Serbian word order is fairly flexible.

The original sentence is a normal, clear order:

• Moje noge su umorne posle treninga.

But you could also hear:

• Noge su mi umorne posle treninga.
• Posle treninga su mi noge umorne.

The meaning stays basically the same, but the focus changes a little depending on what the speaker wants to emphasize.

Usually no.

In standard Serbian, the present tense of biti is normally expressed in sentences like this:

• Noge su umorne.

Leaving out su would sound incomplete or nonstandard in ordinary Serbian. So unlike in some other Slavic languages, you should generally keep it.

Because Serbian does not have articles like a and the.

That means Serbian often relies on context, word order, or other words such as possessives and demonstratives.

So English needs something like:

• my legs
• the legs

But Serbian can simply say:

• noge
• moje noge
• te noge = those legs

In your sentence, definiteness is already clear from moje.

A good approximation is MO-ye.

Important detail:

• Serbian j sounds like English y in yes

So moje is not pronounced like English moyj. It is closer to MO-ye.

A simple learner-friendly approximation is:

MO-ye NO-ge soo oo-MOR-ne POS-le TRE-nin-ga

A few helpful notes:

• j = y sound
• g is always a hard g
• r in Serbian is trilled or tapped more than in English
• stress is not written, so this is only a rough guide

A natural Serbian pronunciation will sound more connected and smoother than the English-style approximation.