Kad sam ponovno susrela domara, pitala sam ga može li nam pomoći s teškim namještajem.

Because Croatian past tense agrees with the gender of the speaker.

Here, the past tense is made with sam + a past participle:

• susrela sam = I met
• pitala sam = I asked

The forms susrela and pitala show that the speaker is female.
If the speaker were male, you would get:

• susreo sam
• pitao sam

So this sentence is specifically something a woman is saying.

Because sam is a clitic, and Croatian clitics tend to go in the second position of their clause.

So:

• Kad sam ponovno susrela domara...
• Pitala sam ga...

In the first clause, kad is the first element, so sam comes right after it.
In the second clause, pitala is the first element, so sam comes after pitala.

This is very normal in Croatian. The same is true of other little words here, such as ga, li, and nam.

Because domara is the accusative singular of the masculine animate noun domar.

The verb susresti takes a direct object, so you need the accusative:

• susrela sam domara = I met the janitor

For masculine animate nouns, the accusative singular usually looks like the genitive singular:

• domar → domara
• student → studenta
• čovjek → čovjeka

If the noun were masculine inanimate, the accusative would usually be the same as the nominative.

Yes. Ponovno means again, and in many situations you can also use opet.

So these are both natural:

• Kad sam ponovno susrela domara...
• Kad sam opet susrela domara...

Very roughly:

• opet is a bit more everyday and common in speech
• ponovno can sound a bit more neutral or slightly more formal

But in this sentence, both work well.

Yes, basically. Susrela is from susresti, and srela is from sresti. These verbs mean the same thing: to meet / come across.

So you may hear both:

• Kad sam ponovno susrela domara...
• Kad sam ponovno srela domara...

Both are normal. The version with sus- is the fuller form; the shorter form is also very common.

Because Croatian often leaves subject pronouns out when they are already clear from the context.

Here, ga already tells you who was asked, and može is third person singular, so on is unnecessary:

• pitala sam ga može li... = I asked him if he could...

You could add on only for emphasis or contrast:

• pitala sam ga može li on pomoći, a ne netko drugi
  = I asked him whether he could help, and not someone else

Normally, though, Croatian prefers to omit the pronoun.

Li is a particle used to form a yes/no question, and it normally comes right after the finite verb.

So:

• Može li...? = Can he...?
• Je li...? = Is he...?
• Ima li...? = Does he have...?

In your sentence, this appears inside an indirect question after pitala sam ga:

• pitala sam ga može li nam pomoći
  = I asked him whether / if he could help us

So literally, Croatian has something like:

• I asked him: can he help us?

Because Croatian does not backshift tenses in the same way English often does.

English commonly says:

• I asked him if he could help us

Croatian often keeps the embedded verb in the present:

• pitala sam ga može li nam pomoći

This is completely normal.

If you used mogao, the meaning would shift more toward was able to, not just a neutral could/can in reported speech. So može li is the natural choice here.

Nam is the dative form of mi = we/us.

The verb pomoći takes the person being helped in the dative:

• pomoći komu? = to help whom?

So:

• pomoći nam = help us
• pomoći mi = help me
• pomoći mu = help him
• pomoći joj = help her

That is why the sentence says može li nam pomoći, not nas pomoći.

Because after the modal verb moći (can / be able to), Croatian uses an infinitive.

So:

• može pomoći = can help
• može doći = can come
• može nositi = can carry

Here:

• može li nam pomoći = can he help us?

Also, pomoći is the perfective verb, which fits well here because the idea is a single act of helping with a specific task.

Because the preposition s / sa meaning with requires the instrumental case.

So both words change:

• težak namještaj → s teškim namještajem

This pattern is very common:

• s prijateljem = with a friend
• s novom knjigom = with a new book
• s teškim namještajem = with heavy furniture

In Croatian, pomoći s čim is a normal way to say help with something.

Because the adjective težak changes its stem in many forms.

Its basic form is:

• težak = heavy

But in many oblique forms, the stem becomes tešk-:

• teškog
• teškom
• teškim

So:

• nominative masculine singular: težak
• instrumental singular: teškim

This is a normal stem alternation in Croatian adjectives of this type.

Yes. Namještaj is normally a mass/collective noun, just like English furniture.

So Croatian usually says:

• težak namještaj
• s teškim namještajem

Even if several pieces are involved, singular namještaj is still natural because it refers to furniture as a whole.

If you specifically wanted to emphasize individual pieces, you could use a different expression, for example:

• s teškim komadima namještaja = with heavy pieces of furniture

To some extent, yes, but not completely freely.

Croatian allows more word order variation than English, but the clitics still have strict placement rules. So this sentence is a very natural neutral version:

• Kad sam ponovno susrela domara, pitala sam ga može li nam pomoći s teškim namještajem.

You could change some parts:

• Kad sam opet srela domara, pitala sam ga može li nam pomoći...

But some changes would be wrong or unnatural because clitics must stay in their usual positions:

• Kad ponovno sam susrela... → wrong
• može nam li pomoći → wrong

So the sentence is flexible in some ways, but words like sam, ga, li, and nam are much less movable than ordinary nouns, adjectives, or adverbs.