Learn Hardcore Russian: Осенью погода то тёплая, то холодная, поэтому я всегда беру с собой шарф. - In autumn the weather is sometimes warm, sometimes cold, so I always take a scarf with me.

Here то … то … is a fixed correlative structure meaning “sometimes … sometimes …”, “now … now …”, or “at one time … at another time …”.

So погода то тёплая, то холодная means:

“The weather is sometimes warm, sometimes cold,”
or “The weather is warm, then cold (and it keeps changing).”

This то is not the same as demonstrative тот (“that”). It is an unstressed little particle used in pairs: то A, то B = the situation alternates between A and B.

Why are тёплая and холодная in the -ая form?

Погода is a feminine noun (it ends in -а and is grammatically feminine).

Adjectives in Russian agree with the noun in gender, number, and case. In the nominative singular:

masculine: тёплый, холодный
feminine: тёплая, холодная
neuter: тёплое, холодное

Because we are describing погода (feminine, singular, nominative), we must use feminine forms: тёплая, холодная.

Why is there no verb like есть or бывает after погода? Shouldn’t it be “погода есть тёплая”?

In present tense descriptive sentences, Russian normally omits the verb “to be” (есть).

So instead of:

Погода есть тёплая. – grammatically possible but sounds unnatural in modern Russian,

people just say:

Погода тёплая. – The weather is warm.

In your sentence, погода то тёплая, то холодная literally is “the weather [is] now warm, now cold,” with the “is” simply left out, which is normal and correct.

What is the function of поэтому here? How is it different from так что?

Поэтому means “therefore / because of that / so” and connects a reason with a result:

Осенью погода то тёплая, то холодная, поэтому я всегда беру с собой шарф.
“In autumn the weather is sometimes warm, sometimes cold, therefore I always take a scarf with me.”

Так что can also mean “so,” but it is a bit more colloquial and often introduces a kind of conclusion or consequence with a more conversational feel:

Осенью погода то тёплая, то холодная, так что я всегда беру с собой шарф.

Both are possible here; поэтому sounds slightly more neutral/formal, так что more chatty.

Why is there a comma before поэтому?

The sentence has two clauses:

Осенью погода то тёплая, то холодная – reason
я всегда беру с собой шарф – result

They are joined by поэтому, which works like a connector (“therefore/so”). Russian punctuation normally separates such clauses with a comma:

[Clause 1], поэтому [Clause 2].

So the comma before поэтому is required.

What tense and aspect is беру? Why not возьму?

Беру is present tense, imperfective aspect of the verb брать (“to take”).

я беру – I take / I am taking (imperfective, present)
я возьму – I will take (perfective, future)

Because we have всегда (“always”), we are talking about a repeated, habitual action. Russian expresses habitual actions with the present/imperfective:

я всегда беру с собой шарф – I always take a scarf with me (as a habit).

Я возьму с собой шарф is a one-time future action: “I will take a scarf (this time).”

Why is it беру с собой шарф and not беру шарф со мной?

Брать / взять с собой (что‑то) is a fixed expression that means “to take something with you”:

Я беру с собой шарф. – I take a scarf with me.

The structure is:

брать / взять
- с собой
  - [object]

You almost never say шарф со мной for this meaning. Шарф со мной would be more like “the scarf is with me” (stating presence), not “I am taking it with me.” For the idea of taking along, always use с собой.

What does с собой literally mean? Why not just со мной?

С собой literally is “with oneself”. It uses the reflexive pronoun себя in the instrumental:

с собой = “with myself / yourself / himself …” (it’s the same form for all persons)

This is part of the idiom брать с собой / взять с собой (to take along with oneself).

Со мной literally means “with me” and is used in different constructions, e.g.:

Пойдёшь со мной? – Will you go with me?

But for “take something with you,” the idiomatic pattern is с собой, not со мной.

What case is шарф in? It looks like nominative.

Шарф is in the accusative singular, as the direct object of беру:

я (кто?) – nominative, subject
беру (что?) шарф – accusative, object

For inanimate masculine nouns like шарф, the accusative singular form equals the nominative singular:

nominative: шарф
accusative: шарф

So it looks the same, but its function in the sentence (object of the verb) tells you it’s accusative.

Could we say погода осенью то тёплая, то холодная instead? Does the word order matter?

Yes, you can say:

Погода осенью то тёплая, то холодная.

Both Осенью погода… and Погода осенью… are grammatically correct. The difference is focus:

Осенью погода… – puts a bit more emphasis on “in autumn” (as opposed to other seasons).
Погода осенью… – starts with “the weather”, then specifies when.

Russian word order is flexible; speakers often move time/place expressions to the beginning to set the scene.

Can the pronoun я be omitted here: … поэтому всегда беру с собой шарф?

Yes, you can omit я:

… поэтому всегда беру с собой шарф.

Russian often drops subject pronouns when it’s clear from the verb ending who is doing the action. Беру clearly indicates 1st person singular, so grammatically я is optional.

Including я can slightly emphasize “I always take a scarf (not someone else)” or just sound a bit more explicit or careful in style, which is normal in neutral speech.

Is there any difference between то тёплая, то холодная and иногда тёплая, иногда холодная?

They are similar but not identical in nuance:

иногда тёплая, иногда холодная – simply “sometimes warm, sometimes cold,” stressing frequency.
то тёплая, то холодная – suggests more of an alternation or back-and-forth change, often a bit more vivid, like “it keeps switching between warm and cold.”

In most contexts you can use either, but то … то … is more idiomatic for describing changeable, alternating states.