Susjedu viđam gotovo svaki dan u liftu, ali njezinu kćer viđam samo vikendom.

Because susjedu is the accusative singular of susjeda, which means female neighbor.

So in this sentence:

• susjeda = a female neighbor
• susjedu = the female neighbor (as the direct object of viđam)

This is important because viđati / vidjeti takes a direct object, and that object usually goes in the accusative.

Compare:

• susjed = male neighbor
• susjeda = female neighbor
• vidim susjeda = I see the male neighbor
• viđam susjedu = I keep seeing / I see regularly the female neighbor

So this sentence is talking about a female neighbor, not a male one.

Viđam suggests something that happens repeatedly, habitually, or from time to time over a period of time.

That fits this sentence very well, because it says:

• gotovo svaki dan = almost every day
• samo vikendom = only on weekends

So the idea is not I see her right now, but rather I keep seeing her / I see her regularly.

A useful contrast:

• vidim = I see / I can see / I saw (depending on context and aspect pairings)
• viđam = I see repeatedly, I keep running into, I see from time to time

So:

• Susjedu viđam gotovo svaki dan = I see the neighbor almost every day
  not
• Susjedu vidim gotovo svaki dan — this is less natural for this repeated, habitual meaning

Viđam is an imperfective frequentative form related to vidjeti.

In practical terms, learners can think of it like this:

• vidjeti = to see
• viđati = to be seeing regularly / to keep seeing / to run into repeatedly

Examples:

• Vidim Marka. = I see Marko.
• Viđam Marka u gradu. = I often see Marko around town.

So in your sentence, viđam is perfect because the speaker is talking about a repeated pattern:

• the neighbor: almost every day
• the daughter: only on weekends

Njezinu kćer is also in the accusative singular, because it is the direct object of viđam.

Breakdown:

• njezina kći = her daughter
  
  • njezina = her
  • kći = daughter

But in the accusative:

• njezinu kćer = her daughter

So:

• viđam njezinu kćer = I see her daughter

The noun kći is somewhat irregular, and in the accusative singular it often appears as kćer.

Yes, essentially.

Both mean her, and both can be used before a feminine noun in the appropriate case. In standard Croatian:

• njezin / njezina / njezino is often considered more formal or more standard
• njen / njena / njeno is very common in everyday speech

So:

• njezinu kćer
• njenu kćer

Both mean her daughter.

In your sentence, njezinu sounds a bit more standard or careful.

Because kći changes form when declined.

This noun is irregular enough that learners usually just have to get used to its most common forms. The important point here is:

• kći = nominative singular
• kćer = accusative singular

So:

• Njezina kći dolazi. = Her daughter is coming.
• Viđam njezinu kćer. = I see her daughter.

Since kćer is the object of viđam, the accusative is required.

Because u can take different cases depending on whether it means:

• location → locative
• movement toward somewhere → accusative

Here it means in the elevator, a location, so Croatian uses the locative:

• lift = elevator
• u liftu = in the elevator

Compare:

• U liftu je. = He/She is in the elevator.
• Ulazi u lift. = He/She is entering the elevator.

So:

• u liftu = location
• u lift = motion toward the inside

Vikendom is an adverbial use of the instrumental form, and it means something like:

• on weekends
• during weekends
• at weekends

This is a very common Croatian pattern with expressions of time.

Examples:

• nedjeljom = on Sundays
• ljeti = in summer
• vikendom = on weekends

So:

• viđam samo vikendom = I see her only on weekends

You do not need a preposition like on here, because Croatian often expresses time directly with a case form.

Because Croatian often expresses time without a preposition.

English says:

• on weekends
• on Sundays
• in winter

Croatian often just uses a case form or adverbial form:

• vikendom
• nedjeljom
• zimi

So vikendom by itself already carries the meaning on weekends.

It means almost every day.

Word by word:

• gotovo = almost
• svaki dan = every day

So:

• gotovo svaki dan = almost every day

This is a very natural expression in Croatian.

Both are possible, but the version in your sentence gives a certain focus or contrast.

Croatian word order is more flexible than English word order. Putting Susjedu first highlights the person being talked about, and it works especially well because the second half also begins with another object:

• Susjedu viđam gotovo svaki dan...
• ali njezinu kćer viđam samo vikendom.

This creates a neat contrast:

• the neighbor — almost every day
• her daughter — only on weekends

If you said:

• Viđam susjedu gotovo svaki dan u liftu...

that would also be grammatical, but it would sound a little less contrastive and less shaped for comparison.

Because Croatian often repeats the verb for clarity and balance, especially in contrasted sentences.

Here the structure is:

• Susjedu viđam gotovo svaki dan u liftu
• ali njezinu kćer viđam samo vikendom

Repeating viđam makes the contrast very clear and natural.

You could sometimes omit repeated material in Croatian if the meaning is obvious, but here repetition sounds better and more complete.

Yes. Ali means but and introduces a contrast.

In this sentence, the contrast is between:

• seeing the neighbor very often
• seeing her daughter only on weekends

So ali is doing exactly the same basic job as English but.

Yes, that is the natural interpretation.

The phrase njezinu kćer means her daughter, and in context her refers back to susjedu.

So the sentence naturally means:

• I see the female neighbor almost every day in the elevator, but I see her daughter only on weekends.

Yes. Lift is a normal and very common Croatian word meaning elevator.

It declines like a regular masculine noun:

• lift = nominative
• u liftu = in the elevator

So this part of the sentence is very ordinary everyday Croatian.

Often, yes.

Depending on context, viđam can mean:

• see
• keep seeing
• see regularly
• run into

In this sentence, a natural English idea is:

• I run into the neighbor almost every day in the elevator...

That captures the repeated, casual nature of viđam quite well.