Na ulazu je veliki izlog, a u izlogu vidim novu posteljinu na rasprodaji.

Croatian does not need a dummy subject like English there.

So instead of saying something like there is, Croatian often simply uses:

• a location phrase
• • je / su
• • the noun

So Na ulazu je veliki izlog is literally closer to At the entrance is a large display window, but in natural English we translate it as There is a large display window at the entrance.

This is about the meaning of the prepositions:

• na = on / at
• u = in / inside

So:

• na ulazu = at the entrance
• u izlogu = in the display window

Croatian chooses the preposition based on how the place is conceptualized. An entrance is treated as a point or area at which something is located, while a display window is treated as a space in which something can be seen.

Because both are in the locative singular.

After na and u, when you are talking about a fixed location, Croatian normally uses the locative case.

Here:

• ulaz → na ulazu
• izlog → u izlogu

Both ulaz and izlog are masculine nouns, and a very common locative singular ending for masculine nouns is -u.

Because they are in different cases and have different grammatical roles.

1. veliki izlog

This is the thing that is there, so it appears in the nominative:

• veliki = nominative masculine singular
• izlog = nominative masculine singular

2. novu posteljinu

This is the direct object of vidim (I see), so it goes into the accusative:

• nova → novu in the accusative feminine singular
• posteljina → posteljinu in the accusative feminine singular

So the adjective changes to match the noun in gender, number, and case.

Because vidim means I see, and the thing you see is the direct object.

In Croatian, direct objects usually go in the accusative case.

So:

• vidim što? = what do I see?
• novu posteljinu

That is why the noun changes from posteljina to posteljinu.

Izlog usually means a shop window or display window, not just any ordinary window.

So in this sentence, it suggests the window where goods are displayed for customers to look at from outside.

That is why u izlogu vidim... means something like I see ... in the display window.

Because je is a clitic, and Croatian clitics usually come in second position in the sentence or clause.

So:

• Na ulazu je veliki izlog

Here, Na ulazu is the first unit, and je comes right after it.

This is very normal Croatian word order. English speakers often expect je to stand earlier, but Croatian clitics follow their own placement rules.

A often translates as and, but it is not exactly the same as i.

• i = simple and
• a = and / while / whereas / and then, often with a slight contrast or shift of focus

In this sentence, a links the two parts smoothly:

• Na ulazu je veliki izlog, a u izlogu vidim...

It feels a bit like:

• There is a large display window at the entrance, and in it I see...

So a is very natural here because it moves from one observation to the next.

Na rasprodaji is a fixed, very common expression meaning on sale or in a sale.

Literally, it is something like at a sale / on discount sale.

Grammatically:

• rasprodaja is a feminine noun
• after na with a static meaning, Croatian uses the locative
• so rasprodaja → rasprodaji

This whole phrase is best learned as a chunk:

• biti na rasprodaji = to be on sale

Grammatically, posteljina is singular.

But in meaning, it often refers to a set or category of bedding/bed linen, so in English it may feel more collective or mass-like.

That is why Croatian can say:

• nova posteljina
  even though English might say:
• new bedding
• new bed linen
• sometimes even new sheets, depending on context

So yes, it is singular in form, even if the real-world meaning can involve multiple items.

Yes, Croatian word order is flexible, but different orders sound more or less natural depending on the context.

The given sentence is very natural because it presents:

1. the location
2. what is there
3. then what the speaker sees in it

You could rearrange parts, but the emphasis would change. For example:

• Veliki izlog je na ulazu...
  puts more focus on the large display window

• U izlogu vidim novu posteljinu...
  starts directly with the location of what is seen

So the original order is a good neutral, descriptive order.

Because Croatian has no articles.

English says:

• a large display window
• the display window
• new bedding

Croatian simply uses the noun and relies on context:

• veliki izlog
• u izlogu
• nova posteljina

So learners have to get used to the fact that Croatian does not mark definiteness with separate words like a or the.

Vidim is the 1st person singular present tense of vidjeti or videti depending on the standard/variant.

It means I see.

So:

• vidim = I see
• vidiš = you see
• vidi = he/she sees

In this sentence, the speaker is describing what they see in the display window right now, so the present tense is the natural choice.