Learn Hardcore Russian: Я иду на рынок пешком, потому что погода тёплая. - I am going to the market on foot because the weather is warm.

Идти → иду is used for a single trip happening now or in the near future: I’m going (right now / this time).
Ходить → хожу usually means repeated/regular trips or the general ability/habit: I go (often / generally).
So Я иду на рынок = this specific trip to the market.

Does Я иду mean “I am going” even though there’s no word for am?

Yes. Russian present tense doesn’t use a separate verb to be (am/is/are) in ordinary sentences. The verb ending in иду already marks I and present tense, so Я иду naturally translates as I’m going / I go (right now).

Why is there a comma before потому что?

Because потому что погода тёплая is a subordinate clause giving the reason. In Russian, a subordinate clause is normally separated by a comma from the main clause:
Я иду..., потому что...

Can I change the word order, like putting потому что... first?

Yes, both are common:

Я иду на рынок пешком, потому что погода тёплая.
Потому что погода тёплая, я иду на рынок пешком.
If the reason clause comes first, you still use a comma after it.

Why is it на рынок and not в рынок?

With destinations, Russian often uses:

в + Accusative for “into” an enclosed place (e.g., в магазин, в дом)
на + Accusative for places seen as an “area/surface/event/institution,” including many public places: на рынок, на почту, на стадион
So на рынок is the standard way to say “to the market.”

What case is рынок in here, and why does it change to рынок?

It’s Accusative singular after на when expressing motion toward a destination.
рынок (nominative) and рынок (accusative) look the same because it’s an inanimate masculine noun; many such nouns don’t change form in the accusative.

What does пешком mean grammatically? Is it a noun or an adverb?

Пешком means on foot / walking. It functions like an adverb (a fixed form meaning “by foot”). You usually place it near the verb: иду пешком, but it can also appear after the destination: иду на рынок пешком.

Could I say Я пешком иду на рынок instead?

Yes. Word order is flexible and changes emphasis:

Я иду на рынок пешком = neutral
Я пешком иду на рынок = emphasizes on foot (as opposed to by bus, etc.)
Пешком я иду на рынок = even stronger emphasis on on foot

Why is погода тёплая with an adjective ending -ая?

Because погода is feminine singular, and adjectives agree with the noun in gender/number/case. In the nominative feminine singular, тёплый becomes тёплая.

Is there an omitted есть (“is”) in погода тёплая?

In present tense, Russian normally omits есть in simple statements:
погода тёплая = “the weather is warm.”
You might see есть in special contexts (contrast, emphasis, definitions), but it’s not typical here.

What’s the difference between потому что and так как?

Both mean “because.”

потому что is very common in everyday speech and feels direct.
так как can sound a bit more formal/bookish or logical (“since/as”).
Your sentence works well with either, but потому что is the most straightforward.

Why is ё used in тёплая, and can it be written as е?

The correct spelling is тёплая with ё, and it’s pronounced tyóplaya (stress on ё). In many texts, ё is often printed as е (теплая), but the pronunciation and stress stay the same. In learning materials, ё is usually shown to help you pronounce correctly.