On uvijek nosi rezervni ključ sa sobom, za svaki slučaj.

Yes. Croatian often drops subject pronouns because the verb ending already shows the person.

• (On) uvijek nosi rezervni ključ sa sobom.

Both versions can mean He always carries a spare key with him.

Keeping on is common when you want to:

• emphasize he
• contrast him with someone else
• make the subject extra clear

So the version with On is correct, but without it would also sound natural in many contexts.

Uvijek is an adverb meaning always. In this sentence, On uvijek nosi... is a very natural neutral word order.

Croatian word order is flexible, but not random. The placement here sounds smooth and standard:

• On uvijek nosi rezervni ključ sa sobom.

Other orders are possible, for example:

• Uvijek nosi rezervni ključ sa sobom. = He always carries a spare key with him.
• On nosi rezervni ključ uvijek sa sobom. = possible, but less neutral

So uvijek after the subject is just the most straightforward everyday choice here.

It can mean both, depending on context.

Nositi commonly means:

• to carry
• to wear

In this sentence, because the object is rezervni ključ (a spare key), the meaning is clearly carry:

• nosi ključ = carries a key

If the object were clothing, then it would mean wear:

• nosi kaput = wears a coat

So the verb itself is the same; the context tells you which meaning is intended.

Because Croatian uses the present tense for regular habits, just like English does in sentences such as He always carries a spare key.

Here nosi is the 3rd person singular present of nositi:

• ja nosim
• ti nosiš
• on/ona/ono nosi

With uvijek, the present tense naturally expresses a repeated or habitual action:

• On uvijek nosi... = He always carries...

Rezervni ključ is the direct object of nosi, so it is in the accusative case.

However, for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular. That is what happens here:

• nominative: rezervni ključ
• accusative: rezervni ključ

So even though the case is accusative, the form does not change.

Also, the adjective must agree with the noun:

• rezervni = masculine singular
• ključ = masculine singular

Because the key is with the same person who is the subject of the sentence. Croatian normally uses the reflexive form in that situation.

• sa sobom literally means with himself / herself / oneself
• here it means with him, referring back to on

So:

• On nosi ključ sa sobom. = He carries the key with him.

If you said s njim, it would usually refer to another male person, not back to the subject:

• On nosi ključ s njim. would sound like He carries the key with him where him is someone else

That is why sa sobom is the natural choice here.

Both s and sa mean with.

Sa is used instead of s when it sounds easier or more natural to pronounce, especially before certain consonant clusters. That is why Croatian says:

• sa sobom not normally
• s sobom

So this is mainly a pronunciation and usage issue. In this phrase, sa sobom is the normal form.

Sobom is the instrumental form of the reflexive pronoun.

After s/sa meaning with, Croatian uses the instrumental:

• sa mnom = with me
• s tobom = with you
• s njim = with him
• sa sobom = with oneself / with himself / herself

So in this sentence:

• sa sobom = preposition sa
  • instrumental sobom

Literally, it means something like for every case. Idiomatically, it means:

• just in case
• for safety
• as a precaution

It is a very common fixed expression in Croatian.

Examples:

• Ponesi kišobran za svaki slučaj. = Take an umbrella just in case.
• Uzeo sam lijekove za svaki slučaj. = I took the medicine just in case.

In your sentence, it explains why he carries the spare key:

• ...za svaki slučaj = ...just in case

Because the expression uses the preposition za, and here za takes the accusative.

So:

• svaki = accusative masculine singular
• slučaj = accusative masculine singular

Since slučaj is masculine inanimate, its accusative singular looks the same as the nominative singular, just like ključ.

This whole phrase is basically a set expression:

• za svaki slučaj = just in case

Learners usually do best by remembering it as one chunk.

Not always. The comma here helps mark za svaki slučaj as an added comment or afterthought.

So both are possible:

• On uvijek nosi rezervni ključ sa sobom, za svaki slučaj.
• On uvijek nosi rezervni ključ sa sobom za svaki slučaj.

With the comma, the phrase feels a bit more parenthetical:

• He always carries a spare key with him, just in case.

Without the comma, it feels slightly more tightly connected to the sentence.

So the comma is often a matter of style and rhythm here, not a major grammar difference.

It is flexible, but some orders sound more natural than others.

Very natural:

• On uvijek nosi rezervni ključ sa sobom.
• Uvijek nosi rezervni ključ sa sobom.

Also possible:

• Rezervni ključ uvijek nosi sa sobom.
• On sa sobom uvijek nosi rezervni ključ.

These are grammatically possible, but they may sound more marked, more literary, or more focused on a particular part of the sentence.

As a learner, the safest neutral order is:

• On uvijek nosi rezervni ključ sa sobom, za svaki slučaj.

Sometimes, yes, but they are not always identical.

• uvijek = always
• stalno = constantly / all the time

In many sentences they overlap, but uvijek is the better choice for a repeated habit like this:

• On uvijek nosi rezervni ključ sa sobom.

If you used stalno, it could sound a bit more like:

• he keeps carrying it around all the time
• it is a constant ongoing pattern

So uvijek is the most natural word here for always.