Learn Hardcore Russian: Не забывай шапку, когда выходишь из дома зимой. - Don’t forget your hat when you leave the house in winter.

забывай = imperative, imperfective → usually implies a general/repeated habit: don’t (ever) forget / don’t make a habit of forgetting.
не negates the imperative: не забывай = don’t forget.

What’s the difference between не забывай and не забудь?

Both can translate as don’t forget, but aspect changes the nuance:

не забывай (imperfective) → a general instruction or repeated situations: Don’t forget (as a rule / whenever this happens).
не забудь (perfective) → a one-time, specific reminder about a particular instance: Don’t forget (this time / right now).
In this sentence, когда выходишь… зимой describes a recurring situation, so не забывай fits well.

Why is шапку in that form? Why not шапка?

Because забывать/забыть takes a direct object in the accusative case.

Dictionary form (nominative): шапка
Accusative singular: шапку
So не забывай шапку = don’t forget the hat.

Is the word свою (my/your own) missing? Should it be не забывай свою шапку?

It can be added, but it’s often unnecessary. In Russian, possession is frequently understood from context, especially with everyday items.

Natural and common: Не забывай шапку…
More explicit/emphatic: Не забывай свою шапку… (e.g., contrasting with someone else’s hat)

Why does Russian use когда выходишь (present tense) to mean a repeated action, not a future form?

In Russian, present tense of an imperfective verb often expresses habitual/repeated actions, especially with когда (when):

когда выходишь = when(ever) you go out / when you leave (as a general situation)
It’s not about one future event; it’s a general rule.

What is the form выходишь exactly?

Выходишь is 2nd person singular present tense of выходить (imperfective):

я выхожу
ты выходишь
он/она выходит
It matches the implied ты in не забывай.

Why is there no subject like ты in the sentence?

Russian often omits personal pronouns when the verb ending already shows the person/number.
Here, both забывай (imperative 2sg) and выходишь (present 2sg) clearly point to ты, so ты is optional and usually only added for emphasis: Ты не забывай…

Why is there a comma before когда?

Because когда выходишь из дома зимой is a subordinate clause (a dependent clause), and Russian normally separates such clauses with a comma:
Не забывай шапку, когда выходишь…
(Comparable to English punctuation in many cases.)

Why is it из дома and not из дом or из доме?

The preposition из (out of/from inside) requires the genitive case.

дом (nominative) → дома (genitive singular)
So из дома = out of the house / from home.

What case is зимой, and why is it used?

Зимой is instrumental case of зима. Instrumental is commonly used to mean at/in (a season) as an adverbial time expression:

зима → зимой = in winter / during winter
Similarly: летом (in summer), весной (in spring), осенью (in autumn).

Why isn’t it в зимой or в зиму?

Russian has two common ways to express in winter:
1) зимой (instrumental) — very common, neutral, like an adverb (during winter).
2) зимой is preferred here because it’s a general seasonal setting.
Forms like в зиму can occur but usually have different meanings/contexts (often motion into a period, or stylistic/poetic), and в зимой is ungrammatical.

Can the word order be changed, and would it sound different?

Yes, Russian word order is flexible, but changes shift emphasis:

Neutral: Не забывай шапку, когда выходишь из дома зимой.
Emphasis on time/condition: Когда выходишь из дома зимой, не забывай шапку.
Both are correct; the second foregrounds the when clause.