Da nismo imali pomoć domara, selidba bi bila toliko naporna da bismo odustali.

Yes. In this sentence, da means if.

Here da nismo imali pomoć domara introduces an unreal / counterfactual condition in the past:

• Da nismo imali... = If we hadn’t had...

This is a very common Croatian pattern for counterfactuals:

• Da + past form in the if-clause
• bi / bismo / biste... in the main clause

So the whole structure is roughly:

• Da nismo imali pomoć domara, selidba bi bila...
• If we hadn’t had the janitor’s help, the move would have been...

In other words, this is not talking about a real past possibility, but about an imagined alternative past.

Because Croatian usually forms this kind of past unreal condition with:

• da + perfect in the condition clause
• conditional in the result clause

So:

• Da nismo imali pomoć domara... = If we hadn’t had the janitor’s help...
• ...selidba bi bila... = ...the move would have been...

A learner might expect both clauses to use something like would, but Croatian does not mirror English that way.

So the division is:

• condition clause: da nismo imali
• main/result clause: bi bila, bismo odustali

That is the normal idiomatic pattern here.

They mark the conditional mood.

Here:

• bi bila = would have been
• bismo odustali = we would have given up

The forms come from the auxiliary biti in the conditional:

• ja bih
• ti bi
• on/ona/ono bi
• mi bismo
• vi biste
• oni/one/ona bi

So in your sentence:

• selidba bi bila
  The subject is selidba (singular), so Croatian uses bi.

• da bismo odustali
  The subject is we, so Croatian uses bismo.

Even though English uses would everywhere, Croatian changes the auxiliary depending on person and number.

Because the implied subject is we.

The sentence does not explicitly say mi, but it is understood from the verb forms:

• nismo imali = we didn’t have
• bismo odustali = we would give up / would have given up

The participles are therefore plural:

• imali
• odustali

If the speaker were a woman speaking only about herself, you would get singular feminine forms in the right context. But here the forms clearly show a first person plural subject.

Because selidba is a feminine singular noun.

In Croatian, the participle agrees with the subject in gender and number:

• selidba = feminine singular
• therefore: bila

Compare:

• problem bi bio = the problem would be
• situacija bi bila = the situation would be
• troškovi bi bili = the costs would be

So bila agrees with selidba, not with the people moving.

Domara is genitive singular.

The noun pomoć often takes a noun in the genitive to express whose help it is:

• pomoć domara = the janitor’s help / help from the janitor
• literally: help of the janitor

So:

• pomoć = help
• domar = janitor / caretaker
• domara = genitive singular of domar

This is very normal Croatian syntax. Similar examples:

• savjet prijatelja = a friend’s advice
• podrška obitelji = the family’s support
• pomoć susjeda = the neighbors’ help

Yes, but the structure would be different.

Your sentence says:

• Da nismo imali pomoć domara...
• literally: If we hadn’t had the janitor’s help...

A possible alternative is:

• Da nije bilo pomoći domara...
• If there hadn’t been the janitor’s help...

Or:

• Bez pomoći domara...
• Without the janitor’s help...

And you can use uz pomoć domara in a positive statement such as:

• Uz pomoć domara, selidba je bila lakša.
• With the janitor’s help, the move was easier.

So uz pomoć is possible in Croatian, but it does not fit this exact sentence as directly as imali pomoć domara or bez pomoći domara.

It means so ... that ...

This is a very common Croatian pattern:

• toliko naporna da... = so exhausting that...

In the full sentence:

• selidba bi bila toliko naporna da bismo odustali
• the move would have been so exhausting that we would have given up

Other examples:

• Bio je toliko umoran da je zaspao. = He was so tired that he fell asleep.
• Bilo je toliko skupo da nismo kupili ništa. = It was so expensive that we bought nothing.

So here toliko intensifies the adjective naporna.

No. The second da does not mean if.

In:

• toliko naporna da bismo odustali

this da introduces a result clause:

• so exhausting that we would give up / would have given up

So the sentence contains two different uses of da:

1. Da nismo imali pomoć domara...
   = If we hadn’t had the janitor’s help...

2. ...toliko naporna da bismo odustali.
   = ...so exhausting that we would have given up.

Croatian uses da in many ways, so learners often need to identify it from the structure, not translate it the same way every time.

Because Croatian usually does not build this idea the same way English does.

English distinguishes:

• would give up
• would have given up

Croatian often uses the same conditional form:

• bismo odustali

Depending on context, that can correspond to:

• we would give up
• we would have given up

Here the whole sentence is clearly about an unreal past situation, so bismo odustali is naturally understood as:

• we would have given up

So even though English has a more visibly perfect-looking form, Croatian does not need an equivalent of would have + past participle here.

It can mean both, depending on context.

Here selidba refers to the move / the moving process. In English, we might say:

• the move
• moving house
• the moving process

So:

• selidba bi bila toliko naporna...
• the move would have been so exhausting...

This is a very natural noun in Croatian for changing residence and the work connected with it.

Croatian word order is fairly flexible, but the given order is natural and clear.

Original:

• Da nismo imali pomoć domara, selidba bi bila toliko naporna da bismo odustali.

You could rearrange parts for emphasis, for example:

• Selidba bi bila toliko naporna da bismo odustali da nismo imali pomoć domara.

But that sounds less neutral and less elegant in many contexts.

The original order is best because it presents:

1. the condition first
2. the consequence second

That is the clearest structure for learners and also very natural in everyday Croatian.

It is not the most natural choice here.

Croatian normally says:

• pomoć domara

rather than:

• pomoć od domara

Using od here sounds less idiomatic. With pomoć, the genitive without a preposition is the standard way to say whose help it is.

So the natural phrase is:

• pomoć domara

not:

• pomoć od domara

It is mostly neutral to slightly formal, but completely natural.

Nothing in it is strange or overly literary. It is the kind of sentence you might find in:

• careful spoken Croatian
• writing
• explanation or narration

The counterfactual structure with Da nismo imali... bi... is standard Croatian, and selidba, pomoć domara, odustali are all normal vocabulary.

So this is a very useful sentence for learning real Croatian grammar.