Stari madrac je tvrd, pa me ujutro bole leđa.

Croatian has no articles, so there is no separate word for the, a, or an.

So:

• stari madrac can mean the old mattress or an old mattress
• the exact interpretation comes from context

That is completely normal in Croatian.

This is about adjective form and position.

• stari is an adjective placed before the noun madrac
• tvrd is an adjective used after the verb je, as part of the predicate

In Croatian, adjectives before a noun often appear in the longer form like stari, while adjectives after biti (to be) are very often in the short form, like tvrd.

So:

• stari madrac = old mattress
• madrac je tvrd = the mattress is hard

You could also say madrac je star for the mattress is old.

Madrac is in the nominative singular because it is the subject of the first clause:

• Stari madrac je tvrd.

Here, stari madrac is the thing being described, so it stays in the basic dictionary form.

Here pa means something like:

• so
• and so
• therefore
• sometimes just and then, depending on context

In this sentence, it connects the two ideas:

• the mattress is hard
• as a result, the speaker’s back hurts in the morning

So pa is functioning like so in English.

Because the verb boljeti works differently from English to hurt.

In Croatian, the structure is closer to:

• Something hurts me

So in:

• me bole leđa

the word me means me and is the short accusative form of ja.

Compare:

• Ja = I
• mene / me = me
• meni / mi = to me

With boljeti, the person affected is normally expressed with the accusative short form:

• Boli me glava. = My head hurts.
• Bole me leđa. = My back hurts.

Because the grammatical subject is leđa, and leđa is plural.

So Croatian is literally saying:

• My back hurts → more literally The back hurts me
• but grammatically: leđa bole

That is why the verb is:

• bole = they hurt

not:

• boli = it hurts

Because leđa is a plural-only noun in Croatian. This is called pluralia tantum.

Even though English uses singular back, Croatian normally uses plural leđa for the body part.

So you say:

• Bole me leđa. = My back hurts.

not a singular form for this general meaning.

This is something you mostly just have to learn as a vocabulary fact.

Because me is a clitic. Croatian clitics are short unstressed words, and they usually appear near the beginning of the clause, often in second position.

So:

• pa me ujutro bole leđa

is natural because me comes very early in the clause.

This is a very common Croatian pattern. Other clitics behave similarly, such as:

• mi
• ti
• ga
• je
• se

Word order in Croatian is flexible, but clitics have special placement rules.

Because ujutro is a fixed adverb meaning in the morning.

So:

• ujutro = in the morning

It is best learned as a whole expression.

You will also see similar time expressions like:

• danas = today
• sutra = tomorrow
• uvečer = in the evening

In this sentence, ujutro tells you when the back pain happens.

Yes, Croatian word order is fairly flexible, but not completely free.

This sentence has a natural, neutral order:

• Stari madrac je tvrd, pa me ujutro bole leđa.

But you could change the order a bit for emphasis, for example:

• Ujutro me bole leđa. = emphasis on in the morning
• Leđa me bole ujutro. = possible, but with a different rhythm/emphasis

However, you still need to respect clitic placement, so me cannot just go anywhere.

So the given version is a very good, natural model to learn from.

Because pa is connecting two clauses here:

• Stari madrac je tvrd
• pa me ujutro bole leđa

In Croatian, when pa links two independent clauses like this, a comma is normally used.

So the comma helps show the pause and the logical connection between the two parts.