Lekarz mówi, że potrzebuję więcej witaminy D zimą.

Potrzebuję is the 1st person singular form of potrzebować in the present tense, so it means I need.

• ja potrzebuję = I need
• ty potrzebujesz = you need
• on/ona potrzebuje = he/she needs

So in this sentence, the speaker is saying what they need: że potrzebuję... = that I need...

The final -ę is very important because it shows I am the one who needs it.

In Polish, subject pronouns are often left out because the verb ending already shows who the subject is.

So:

• potrzebuję already means I need
• adding ja is possible, but usually only for emphasis

So:

• że potrzebuję więcej witaminy D = that I need more vitamin D
• że ja potrzebuję więcej witaminy D would sound more emphatic, like that I need more vitamin D

This is very common in Polish.

Że means that and introduces a subordinate clause.

So:

• Lekarz mówi = The doctor says
• że potrzebuję więcej witaminy D zimą = that I need more vitamin D in winter

This works very much like English that in sentences such as The doctor says that...

In Polish, że is extremely common after verbs like:

• mówić = to say
• myśleć = to think
• wiedzieć = to know
• uważać = to think/believe

After więcej (more), Polish normally uses the genitive case.

So:

• witamina D = vitamin D (dictionary/basic form)
• więcej witaminy D = more vitamin D

This is a very common pattern:

• więcej czasu = more time
• więcej wody = more water
• więcej pieniędzy = more money

So więcej is one of those words that often causes the following noun to change form.

Witaminy D is in the genitive case.

The basic form is:

• witamina D = vitamin D

But after więcej, we need the genitive:

• więcej witaminy D = more vitamin D

Here, only witamina changes to witaminy. The letter name D stays the same.

So:

• nominative: witamina D
• genitive: witaminy D

Because D here is the name of the vitamin and acts like a fixed label. In expressions like witamina D, witamina C, witamina A, the noun witamina changes when needed, but the letter usually stays the same.

Examples:

• witamina C
• potrzebuję witaminy C
• za mało witaminy A
• dużo witaminy B12

So the grammatical change happens in witamina, not in the vitamin label.

Zimą is the instrumental case form of zima (winter).

In Polish, seasons are often used in the instrumental to mean in / during that season:

• zimą = in winter
• latem = in summer
• jesienią = in autumn
• wiosną = in spring

So:

• zimą = in winter / during winter

This is a fixed and very common time expression.

Yes, w zimie is also possible and means in winter. However, zimą is very common and often sounds a bit more natural in general statements.

So both can work:

• potrzebuję więcej witaminy D zimą
• potrzebuję więcej witaminy D w zimie

The version with zimą is compact and very idiomatic.

Yes, Polish word order is more flexible than English word order because cases and verb endings carry a lot of grammatical information.

The neutral version here is:

• Lekarz mówi, że potrzebuję więcej witaminy D zimą.

But other orders are possible for emphasis, for example:

• Lekarz mówi, że zimą potrzebuję więcej witaminy D.
• Zimą lekarz mówi, że potrzebuję więcej witaminy D.
  This could suggest in winter is when the doctor says it.
• Więcej witaminy D potrzebuję zimą.
  This sounds more marked and emphasizes more vitamin D.

So the original sentence is a natural, neutral order.

In this sentence, Lekarz mówi means The doctor says.

The verb mówić can mean:

• to speak
• to say

The meaning depends on context.

Here, because it is followed by że... (that...), the meaning is clearly says:

• Lekarz mówi, że... = The doctor says that...

If you wanted The doctor is speaking, the context would be different, for example:

• Lekarz mówi do pacjenta. = The doctor is speaking to the patient.

Here więcej works like a quantifier, meaning more.

It is used to talk about a greater quantity of something:

• więcej wody = more water
• więcej czasu = more time
• więcej witaminy D = more vitamin D

It is related to the comparative of dużo/wiele in meaning, but in practical learning terms, it is easiest to treat więcej as the standard word for more before nouns and with quantities.

In Polish, a subordinate clause introduced by że is separated by a comma.

So:

• Lekarz mówi, że...

This is standard punctuation in Polish. In English, that often does not need a comma, but in Polish the comma is normally required before że.

A rough pronunciation guide is:

po-trze-BU-yeh

More carefully:

• po = like po in Poland
• trze = roughly tsheh / cheh with a Polish rz
• bu = boo
• ję at the end sounds roughly like yeh, with a nasal element in careful speech

A useful learner approximation is:

pot-sheh-BOO-yeh

The important thing is that -uję is pronounced something like -oo-yeh.

A rough pronunciation is:

VYEN-tsey

Notes:

• w in Polish sounds like English v
• ię is roughly yen here
• c sounds like ts
• ej sounds like ay in day, but shorter

So więcej is approximately:

VYEN-tsey

Here it is treated like an uncountable substance/quantity, similar to English water, sugar, or vitamin D.

That is why Polish uses:

• więcej witaminy D = more vitamin D

You are not counting separate items; you are talking about a greater amount.

No, not naturally. In this sentence, zimą attaches to the whole idea I need more vitamin D in winter.

So the meaning is:

• During winter, I need more vitamin D

If you wanted a comparison like more vitamin D than in winter, Polish would need a different structure, for example something with niż zimą.

Here are the main dictionary forms:

• lekarz = doctor
• mówić = to say / to speak
• że = that
• potrzebować = to need
• więcej = more
• witamina = vitamin
• zima = winter

Forms used in the sentence:

• Lekarz = dictionary form
• mówi = from mówić
• potrzebuję = from potrzebować
• witaminy = genitive of witamina
• zimą = instrumental of zima

Yes, that is a good literal translation. In natural English, you would usually say:

• The doctor says that I need more vitamin D in winter.
• or simply The doctor says I need more vitamin D in winter.

The Polish sentence does not include a word for the, because Polish has no articles like a and the. Context tells us that lekarz means the doctor here.