Learn Hardcore Russian: Зимой я пью чай с лимоном и витамином C, чтобы по крайней мере не простудиться. - In winter I drink tea with lemon and vitamin C in order at least not to catch a cold.

Зимой is the instrumental singular form of зима (winter), and here it’s used in a very common pattern: instrumental of time = “in/at (a season or part of the day)”.

Russian often uses the bare instrumental (without a preposition) to talk about when something happens, especially:

зимой – in (the) winter
летом – in (the) summer
осенью – in (the) autumn
весной – in (the) spring
утром – in the morning
ночью – at night

So:

Зимой я пью чай… = In winter I drink tea…

Forms with в are possible in other meanings, but sound wrong or unnatural here:

в зиме – literally “in the winter (as inside it)” – used only in special abstract/poetic contexts
в зиму – directional, like “into the winter (season)”, again very rare and stylistic

For a normal “in winter (as a general time)”, you say зимой.

Why is it с лимоном и витамином C, not с лимон or с витамин C?

Because the preposition с (“with”) in the sense of “together with / containing” requires the instrumental case:

с кем? с чем? – with whom? with what?
- с лимоном – with lemon
- с витамином C – with vitamin C

So the base (dictionary) forms are:

лимон → instrumental лимоном
витамин → instrumental витамином

After с in this meaning, you must use instrumental:

чай с молоком – tea with milk
кофе с сахаром – coffee with sugar
суп с грибами – soup with mushrooms
пицца с сыром – pizza with cheese

Using nominative (с лимон, с витамин) would be ungrammatical.

Why is it витамином C? Should the letter C be Cyrillic С or Latin C?

In standard Russian, vitamin C is written витамин C or витамин С:

On paper or in books, you will usually see the Cyrillic letter С (which looks the same as Latin C): витамин С.
On the computer, people often just type a Latin C by habit. It doesn’t change the meaning in everyday text, but strictly speaking, proper Russian text should use Cyrillic.

Grammatically, витамином С is:

витамин – noun, “vitamin”
ом – instrumental ending → витамином
С – the label/name of the vitamin

So витамином С = “with vitamin C” (instrumental after с).

Why is there a comma before чтобы?

In Russian, чтобы usually introduces a subordinate clause (a dependent clause of purpose, result, wish, etc.). Such clauses are normally separated by a comma.

Here:

Зимой я пью чай с лимоном и витамином C,
– main clause
чтобы по крайней мере не простудиться.
– subordinate clause of purpose (“in order not to catch a cold at least”)

So the comma marks the border between the main action and its goal/motive.

Pattern:

Я делаю Х, чтобы сделать Y.
- Я читаю книги, чтобы лучше писать. – I read books in order to write better.
- Я учу русский, чтобы понимать фильмы. – I study Russian to understand movies.

Hence: comma + чтобы is standard in this structure.

Why is it чтобы … не простудиться (infinitive) and not чтобы я не простудился?

Both are grammatically possible, but they are different constructions:

чтобы не простудиться – infinitive of purpose
- Literally: “in order not to catch a cold” (no explicit subject inside the clause)
- The subject is understood to be the same as in the main clause (я).
- This is very common, neutral, and fits perfectly here.
чтобы я не простудился – finite verb (past tense, subjunctive/conditional-like)
- Literally: “so that I don’t (won’t) catch a cold”
- This sounds a bit more like a wish or desired outcome, often used when someone else is acting so that I don’t catch a cold, or in more formal/structured speech.

In everyday usage, for “I do X so that I don’t get sick”, Russians strongly prefer:

Я пью чай …, чтобы не простудиться.

rather than:

Я пью чай …, чтобы я не простудился. (understandable but stylistically odd here)

Why is the verb простудиться (perfective) and not простужаться (imperfective)?

The verb pair is:

простужаться – imperfective: “to be catching colds / to get colds repeatedly / process”
простудиться – perfective: “to catch a cold (once, as a result)”

With чтобы expressing a specific result you want to achieve or avoid (a one-time outcome), Russian normally uses the perfective infinitive:

чтобы выучить слова – to (actually) learn the words
чтобы успеть на поезд – to manage to catch the train
чтобы не опоздать – so as not to be late
чтобы не простудиться – so as not to catch a cold (even once)

Using чтобы не простужаться would shift the meaning toward “in order not to be in a state of catching colds repeatedly / all the time”, which is possible in some contexts, but here, the natural idea is “I don’t want to catch a cold at all (this winter)”, so простудиться (perfective) fits best.

What does the -ся in простудиться do? Why not just простудить?

The suffix -ся (or -сь after vowels) is the reflexive marker. It often means:

you do something to yourself, or
something happens to you / by itself (no direct object)

Compare:

простудить кого-то – to give someone a cold, to cause someone else to catch a cold
простудиться – to catch a cold oneself

So:

Я простудил ребёнка. – I made the child catch a cold (I chilled him).
Я простудился. – I caught a cold (I myself got sick).

In this sentence не простудиться = “not to catch a cold (myself)”, so the reflexive form is required.

What exactly does по крайней мере mean here, and where can it go in the sentence?

По крайней мере is an idiom meaning “at least” (in the sense of “if nothing else / in the worst case”).

In this sentence, it softens the statement:

Without it: … чтобы не простудиться. – so as not to catch a cold.
With it: … чтобы по крайней мере не простудиться. – so that, at least, I don’t catch a cold (even if it doesn’t help with anything else).

Typical uses:

Возьми зонт, по крайней мере не промокнешь. – Take an umbrella, at least you won’t get wet.
Я приду, по крайней мере постараюсь. – I’ll come, or at least I’ll try.

Position in this sentence:

чтобы по крайней мере не простудиться – most natural
чтобы не простудиться по крайней мере – possible, but sounds a bit awkward here

Generally по крайней мере stands before the word or phrase it modifies; here, it modifies the whole не простудиться idea.

Can we change the word order, e.g. Я зимой пью чай… instead of Зимой я пью чай…? What changes?

Both word orders are grammatically correct:

Зимой я пью чай с лимоном и витамином C…
- Emphasis / topic: зимой (as opposed to other seasons).
- Roughly: As for winter / In winter, I drink tea…
Я зимой пью чай с лимоном и витамином C…
- Slightly more neutral; emphasis starts on я (I), then adds зимой as a time detail.
- Roughly: I (in winter) drink tea…

Russian word order is relatively flexible; moving adverbials like зимой can subtly change what’s being contrasted or highlighted, but the basic meaning stays the same.

You could also say:

Я пью зимой чай с лимоном… – less typical, but still possible; it might sound a bit more poetic or stylized.

In everyday speech, Зимой я… and Я зимой… are the most natural.