Ta sukienka to okazja, bo kosztuje mniej niż zwykle.

Because sukienka is a feminine singular noun, and the demonstrative this has to agree with it:

• ten for masculine singular
• ta for feminine singular
• to for neuter singular

So:

• ta sukienka = this dress
• compare: ten płaszcz = this coat
• compare: to krzesło = this chair

It is in the nominative singular.

That is the basic dictionary form of the noun, and it is used here because ta sukienka is the subject of the sentence:

• Ta sukienka to okazja = This dress is a bargain

You are identifying what the dress is, so the noun naming the thing being talked about stays in the nominative.

Here to is not the ordinary demonstrative this/it. It is a very common Polish word used in equational sentences, where one thing is identified as another.

So:

• Ta sukienka to okazja = This dress is a bargain
• To mój brat = This is my brother
• Warszawa to stolica Polski = Warsaw is the capital of Poland

In sentences like this, to works a bit like an equals sign.

Because Polish often uses to instead of jest in this kind of identifying sentence.

So Polish commonly says:

• Ta sukienka to okazja

rather than:

• Ta sukienka jest okazją

Both are possible, but they are structured differently:

• X to Y: very common for definitions, identifications, and simple statements of equivalence
• X jest Y: also possible, but then Y often appears in the instrumental case

So the version with to is very natural here.

Because after the pattern X to Y, Polish usually keeps Y in the nominative:

• Ta sukienka to okazja

If you use jest, then Polish often uses the instrumental:

• Ta sukienka jest okazją

So both forms exist, but the case changes with the structure:

• to + nominative
• jest + often instrumental

In this sentence, okazja means a bargain or a great deal.

The noun okazja can also mean an opportunity in other contexts, so it is a useful word with more than one meaning.

Examples:

• To świetna okazja. = It’s a great opportunity.
• Ta sukienka to okazja. = This dress is a bargain.

So the surrounding context tells you which meaning is intended.

Bo means because. It is very common in everyday Polish and sounds natural in speech.

So:

• Ta sukienka to okazja, bo kosztuje mniej niż zwykle. = This dress is a bargain because it costs less than usual.

You could also use ponieważ, which is more formal or bookish:

• Ta sukienka to okazja, ponieważ kosztuje mniej niż zwykle.

Both are correct. In casual spoken Polish, bo is extremely common.

Kosztuje is the 3rd person singular present tense of the verb kosztować = to cost.

The subject is ta sukienka (this dress), which is singular, so the verb must also be singular:

• sukienka kosztuje = the dress costs

Other forms:

• Ja kosztuję would mean I cost
• One kosztują = They cost

Here the subject is feminine, but in the present tense the verb form kosztuje does not change for gender, only for person and number.

In Polish, subject pronouns are often omitted when they are clear from the context.

So instead of saying:

• Ta sukienka to okazja, bo ona kosztuje mniej niż zwykle

Polish normally just says:

• Ta sukienka to okazja, bo kosztuje mniej niż zwykle

The understood subject of kosztuje is still ta sukienka. English usually needs it, but Polish often leaves it out.

This is a comparative structure:

• mniej = less
• niż = than
• zwykle = usually / as usual

So:

• mniej niż zwykle = less than usual

This is very similar to other Polish comparisons:

• więcej niż teraz = more than now
• szybciej niż wcześniej = faster than before

After comparative words like mniej, więcej, lepiej, gorzej, Polish commonly uses niż.

Zwykle is an adverb meaning usually or as usual.

In the phrase niż zwykle, it means than usual. Polish does not need to add extra words like normally or it usually does. The phrase is naturally understood.

So:

• kosztuje mniej niż zwykle
  literally: it costs less than usually natural English: it costs less than usual

This is a very common and natural Polish expression.

Yes, you can, and it is grammatically correct:

• Ta sukienka jest okazją

But in that version, okazją is in the instrumental case, and the sentence may sound a little less like the fixed everyday retail-style statement This dress is a bargain.

Very often, Polish prefers:

• Ta sukienka to okazja

for this kind of simple identification.

So for a learner, the safest takeaway is:

• X to Y is a very common way to say X is Y
• X jest Y is also possible, but the case pattern often changes

The given word order is the most neutral and natural:

• Ta sukienka to okazja, bo kosztuje mniej niż zwykle.

Polish word order is more flexible than English, but changes usually affect emphasis.

For example, you could move parts around for stylistic reasons, but the basic version is best for learners. The sentence starts with Ta sukienka because that is the topic: we are talking about this dress.

So while Polish can be flexible, the original order is the standard, natural choice here.