Ten bukiet czerwonych róż stoi już w wazonie na stole i ma piękny zapach.

Ten is the masculine singular nominative form of this.
Bukiet is a masculine noun, and here it is the subject of the sentence, so it appears in the nominative case.

So:

• ten = this
• bukiet = bouquet

Together, ten bukiet means this bouquet.

Because after a noun like bukiet (bouquet), Polish usually uses the genitive to show what the bouquet consists of.

So the structure is:

• bukiet czego? = bouquet of what?
• czerwonych róż = of red roses

That is why you do not get nominative czerwone róże here.
Instead, you get genitive plural:

• czerwone róże = red roses
• bukiet czerwonych róż = a bouquet of red roses

Because czerwonych agrees with róż, not with bukiet.

Since róż is in the genitive plural, the adjective must also be in the genitive plural:

• nominative plural: czerwone róże
• genitive plural: czerwonych róż

So czerwonych is simply the correct form of red for of red roses.

Róż is the genitive plural form of róża (rose).

Basic forms:

• singular nominative: róża
• plural nominative: róże
• plural genitive: róż

Because bukiet requires the idea of roses, Polish uses the genitive plural: róż.

Polish often uses position verbs where English might simply use is.

• stoi = is standing
• leży = is lying
• wisi = is hanging

A bouquet in a vase is imagined as standing upright, so stoi is very natural.

So stoi w wazonie means something like:

• is standing in the vase
• more naturally in English: is in the vase

Już usually means already.

Here it suggests that the bouquet has already been placed there, or that it is there now by this point:

• stoi już w wazonie = is already standing in the vase

Polish often places już before the part it highlights, but word order can be somewhat flexible.

This is about Polish prepositions for location.

• w = in
• na = on

So:

• w wazonie = in the vase
• na stole = on the table

Both prepositions here express location, so they are followed by the locative case.

Because after w and na when they mean location, Polish uses the locative case.

So:

• wazon → w wazonie
• stół → na stole

This is a very common pattern in Polish:

• w domu = in the house
• na biurku = on the desk
• w pokoju = in the room

Ma piękny zapach literally means has a beautiful smell / has a lovely scent. It is a very natural Polish way to express that something smells nice.

Breakdown:

• ma = has
• piękny = beautiful / lovely
• zapach = smell / scent

So Polish often uses a noun phrase here instead of an adjective-based structure.

A very close English equivalent would be:

• It has a lovely scent
• or simply It smells beautiful / lovely

Because piękny describes zapach, and zapach is a masculine singular noun.

So the adjective must match it:

• piękny zapach = a beautiful scent

If the noun were feminine or neuter, the adjective would change:

• piękna róża = a beautiful rose
• piękne zdjęcie = a beautiful photo

Polish does not have articles like a or the.

Whether something is a bouquet or the bouquet is understood from context.
In this sentence, ten helps point to a specific bouquet:

• ten bukiet = this bouquet

So Polish uses words like ten, ta, to when it wants to be more specific, but it still does not have a separate article system like English.

Yes. Polish word order is more flexible than English because endings show grammatical relationships.

This sentence has a neutral, natural order:

• Ten bukiet czerwonych róż stoi już w wazonie na stole i ma piękny zapach.

But other orders are possible for emphasis, for example:

• Już stoi ten bukiet czerwonych róż w wazonie na stole...
• W wazonie na stole stoi już ten bukiet czerwonych róż...

These variations may sound more marked or literary, but they are possible.

Yes. The subject is still ten bukiet czerwonych róż.

Polish often does not repeat the subject if it is already clear:

• stoi już w wazonie na stole = is already standing in the vase on the table
• i ma piękny zapach = and has a beautiful scent

So both verbs, stoi and ma, refer to the same bouquet.

Yes, very common.

Polish frequently uses this pattern:

• noun + genitive to show contents, type, or quantity

Examples:

• szklanka wody = a glass of water
• filiżanka herbaty = a cup of tea
• bukiet róż = a bouquet of roses
• kilo jabłek = a kilo of apples

So bukiet czerwonych róż is a very normal and important pattern to learn.