Moja kuzynka mówi, że rajstopy są dziś za cienkie na taką pogodę.

Because kuzynka is a feminine singular noun, and the possessive pronoun has to agree with it in gender, number, and case.

• moja kuzynka = my female cousin
• mój kuzyn = my male cousin

So moja is the correct feminine form here.

Yes. Polish distinguishes the cousin’s gender:

• kuzynka = female cousin
• kuzyn = male cousin

English cousin does not show gender, but Polish usually does.

Mówi is the 3rd person singular present tense of mówić (to speak / to say).

Here it most naturally means says or is saying. Polish present tense often covers both ideas that English separates into simple present and present continuous.

So depending on context, moja kuzynka mówi can mean:

• my cousin says
• my cousin is saying

Because in Polish, a comma is normally used before że when it introduces a subordinate clause.

Here:

• Moja kuzynka mówi = main clause
• że rajstopy są dziś za cienkie na taką pogodę = subordinate clause

So the comma is required in standard Polish spelling.

Że means that and introduces a clause reporting what someone says, thinks, knows, etc.

So:

• mówi, że... = says that...

This is extremely common in Polish:

• Myślę, że... = I think that...
• Wiem, że... = I know that...
• Powiedział, że... = He said that...

Because rajstopy is a noun that is normally used in the plural, just like English tights.

So even when you mean one pair, Polish still says rajstopy, not a regular singular form in everyday use.

That is why the sentence treats it as grammatically plural.

Because the subject rajstopy is grammatically plural, so the verb must also be plural.

• jest = is (singular)
• są = are (plural)

So:

• Rajstopy są cienkie = The tights are thin

Even if you mean one pair of tights, Polish still uses the plural verb.

Here za means too.

So:

• za cienkie = too thin
• za drogie = too expensive
• za późno = too late

This is a very common pattern in Polish: za + adjective/adverb.

A slightly more formal alternative is zbyt:

• zbyt cienkie = too thin

But za cienkie is very natural and common in everyday speech.

Because the adjective has to agree with rajstopy, which is plural and non-masculine-personal.

In nominative plural, adjectives often look like this with things or feminine nouns in the plural:

• cienkie rajstopy
• nowe buty
• małe dzieci (this one is a bit different semantically, but the ending pattern is similar)

Compare:

• cienki sweter = thin sweater
• cienka bluzka = thin blouse
• cienkie rajstopy = thin tights

So cienkie matches rajstopy.

Yes. Dziś means today, and dzisiaj also means today.

They are very close in meaning. Dziś is just the shorter form and is extremely common. In many contexts, they are interchangeable:

• dziś
• dzisiaj

So this sentence could also use dzisiaj without changing the basic meaning.

Here na means something like for or for this kind of.

So za cienkie na taką pogodę means:

• too thin for this kind of weather
• too thin for weather like this

In Polish, na + accusative is often used to talk about suitability or appropriateness:

• To nie jest ubranie na zimę. = This isn’t clothing for winter.
• To dobre na spacer. = This is good for a walk.
• Za lekkie na mróz. = Too light for frost.

So na taką pogodę is a very natural expression here.

Because after na in this meaning, Polish uses the accusative case.

The basic dictionary forms are:

• taka pogoda = such weather / weather like this

But after na, they change to the accusative:

• na taką pogodę

The changes are:

• taka → taką
• pogoda → pogodę

This happens because both words are feminine singular, and the accusative has different endings from the nominative.

Not completely. Polish word order is more flexible than English word order, because endings carry a lot of the grammatical information.

The given sentence is neutral and natural:

• Moja kuzynka mówi, że rajstopy są dziś za cienkie na taką pogodę.

But you could move some parts for emphasis, for example:

• Moja kuzynka mówi, że dziś rajstopy są za cienkie na taką pogodę.
• Dziś moja kuzynka mówi, że rajstopy są za cienkie na taką pogodę.

These versions shift emphasis slightly, but the core meaning stays the same.