Da imam više novca, ljeto bih provela na moru.

Here da introduces a hypothetical condition. In this pattern, it works much like if in English:

• Da imam više novca... = If I had more money...

It does not always mean if in every context, but in this sentence that is its function.

Because Croatian does not copy the English tense pattern here.

In this kind of unreal or imagined condition, Croatian often uses:

• da + present tense in the conditional clause
• a conditional form in the main clause

So:

• Da imam više novca, ... bih ...

means:

• If I had more money, I would ...

The idea of unreality comes from the whole structure, not from putting imam into a past tense.

It is the conditional form of the verb provesti.

It is built from:

• bih = 1st person singular conditional auxiliary
• provela = the l-participle of provesti

So bih provela means I would spend.

The conditional auxiliary forms are:

• bih = I would
• bi = you/he/she/it would
• bismo = we would
• biste = you would
• bi = they would

No. Provela agrees with the subject, not with ljeto.

The subject ja is omitted, but it is understood. Since the form is provela, the speaker is female.

• female speaker: provela
• male speaker: proveo

So:

• Da imam više novca, ljeto bih provela na moru. = said by a woman
• Da imam više novca, ljeto bih proveo na moru. = said by a man

It does not agree with ljeto, even though ljeto is neuter.

Because Croatian very often drops subject pronouns when they are already clear from the verb form.

Here, several things show the subject clearly:

• imam = first person singular
• bih = first person singular
• provela = singular, feminine

So ja is unnecessary unless you want extra emphasis.

Novca is in the genitive singular.

After words like više meaning more, Croatian normally uses the genitive:

• više vremena = more time
• više ljudi = more people
• više novca = more money

Here novac is treated as a mass noun, so više novca is the standard form.

Because novac here means money as an uncountable substance, like English money, not coins or separate items.

So Croatian normally says:

• više novca = more money

not a regular countable plural meaning.

Ljeto is the direct object of provesti, so it is in the accusative.

Croatian says literally:

• provesti ljeto = to spend the summer

No preposition is needed.

Also, ljeto is a neuter noun, and in the singular its nominative and accusative forms look the same, so you see ljeto either way.

Because provesti is perfective, and it fits the idea of spending a complete, defined period of time.

• provesti ljeto = spend the summer as a whole
• provoditi is imperfective and would sound more ongoing, repeated, or habitual

So provela bih ljeto is the natural choice when talking about one imagined summer as a complete period.

Because na moru means at the seaside / by the sea / on the coast / on holiday at the sea.

• na moru = at the seaside
• u moru = in the sea, physically in the water

So in this sentence na moru is the idiomatic choice.

It is locative singular after na.

In this expression:

• na moru

the meaning is location, so na takes the locative.

Because bih is a clitic, and Croatian clitics usually go in second position in their clause.

In the main clause, the first element is ljeto, so the clitic comes right after it:

• ljeto bih provela na moru

That word order is very natural.

Yes. Croatian word order is flexible, though not completely free.

For example, you could also say:

• Da imam više novca, provela bih ljeto na moru.

This is also correct. The difference is mainly one of emphasis and information focus.

• ljeto bih provela... puts a bit more focus on the summer
• provela bih ljeto... is more neutral for many speakers

Yes. It corresponds closely to the English second conditional:

• If I had more money, I would spend the summer at the seaside.

So it expresses an unreal, imagined, or unlikely present situation and its result.

Yes. That is a very common alternative:

• Kad bih imala više novca, provela bih ljeto na moru.

Compared with da imam, kad bih imala can sound a bit more straightforward as a hypothetical condition.

Da imam... can sometimes feel slightly more wish-like, depending on context.

Yes. On its own, Da imam više novca! can mean something like:

• If only I had more money!
• I wish I had more money!

So da-clauses can have a slightly wishful flavor, especially when they stand alone. In your full sentence, though, it clearly functions as the condition for the main clause.