Зимой я ношу тёплый шарф и варежки.

I

Breakdown of Зимой я ношу тёплый шарф и варежки.

• и: and
• зимой: in winter
• тёплый: warm

Questions & Answers about Зимой я ношу тёплый шарф и варежки.

Why is зимой used for in winter? What case is it?

Зимой is the instrumental singular of зима (winter), but here it is being used adverbially to mean in winter / during winter.

This is very common with seasons in Russian:

• зимой — in winter
• весной — in spring
• летом — in summer
• осенью — in autumn

So Russian often says simply зимой, without a preposition, where English uses in.

Do we need я here, or could it be left out?

You can leave it out.

Russian often drops subject pronouns when they are clear from the verb ending.
Ношу already means I wear / I carry, so:

• Зимой я ношу тёплый шарф и варежки.
• Зимой ношу тёплый шарф и варежки.

Both are natural.

Including я can make the subject a little clearer or more emphatic, especially if you are contrasting yourself with someone else.

Why is the verb ношу used here instead of надеваю?