Если идёт дождь, я беру зонт.

This is just a standard Russian way to say it is raining or rain is falling.

• идёт = goes / is going
• дождь = rain

In weather expressions, Russian often uses verbs in ways that do not match English literally. So идёт дождь is a normal idiomatic expression meaning it’s raining.

You may also hear:

• дождь идёт
• идёт снег = it’s snowing

So even though the literal wording feels strange to an English speaker, this is completely natural Russian.

Yes — and that is exactly why если can feel tricky here.

In this sentence, если literally means if, but in context it often expresses a general repeated condition, which English may translate more naturally as when or whenever.

So:

• Если идёт дождь, я беру зонт.
• Literally: If it’s raining, I take an umbrella.
• Natural sense: When/whenever it rains, I take an umbrella.

Russian commonly uses если in this kind of habitual statement. It does not always mean a doubtful or hypothetical if in the English sense.

Because the sentence describes a habitual, general, or repeated situation.

• идёт = is raining / rains
• беру = I take

Russian uses the present tense here the same way English does in sentences like:

• If it rains, I take an umbrella.
• When it rains, I take an umbrella.

This is not about one specific moment right now. It means something like this is what I usually do in that situation.

Because беру is imperfective, and it fits a habitual repeated action.

• брать → беру = imperfective, used for repeated, habitual, ongoing actions
• взять → возьму = perfective, used for a single completed action

Here the speaker means:

• Whenever it rains, I take an umbrella.

That is a repeated habit, so беру is the natural choice.

If you said Если пойдёт дождь, я возьму зонт, that would mean something more like:

• If it starts raining, I’ll take an umbrella.

That refers to a specific future situation, not a general habit.

Зонт is in the accusative case because it is the direct object of беру:

• я беру что? → зонт

However, for inanimate masculine singular nouns, the accusative form is usually the same as the nominative form.

So:

• nominative: зонт
• accusative: зонт

That is why you do not see a visible ending change here.

Russian has no articles — no equivalent of a, an, or the.

So зонт can mean:

• an umbrella
• the umbrella
• sometimes just umbrella in a general sense

The exact meaning depends on context.

In this sentence, English naturally uses an umbrella, but Russian simply says зонт.

Yes.

• зонт = umbrella (neutral standard word)
• зонтик = little umbrella, often used as a common everyday word too

Зонтик is a diminutive form, so it can sound:

• more affectionate
• more conversational
• sometimes slightly softer or more informal

So both are possible, but зонт is the more neutral dictionary-style choice.

Because Если идёт дождь is a subordinate clause, and in Russian subordinate clauses are normally separated by a comma.

Structure:

• Если идёт дождь, = If it’s raining,
• я беру зонт. = I take an umbrella.

Russian punctuation is quite strict here, so the comma is required.

Yes, absolutely.

Both are correct:

• Если идёт дождь, я беру зонт.
• Я беру зонт, если идёт дождь.

The meaning stays basically the same. The difference is mainly about emphasis and information flow.

• Если идёт дождь, я беру зонт.
  Starts with the condition: If it’s raining...
• Я беру зонт, если идёт дождь.
  Starts with what I do: I take an umbrella...

Russian word order is flexible, but the comma is still needed.

Yes, it is grammatically possible, but Если идёт дождь is the more neutral and natural order here.

Both mean the same thing, but:

• идёт дождь sounds like the usual weather expression
• дождь идёт may sound more marked, as if дождь is being emphasized or treated as the topic

So for a learner, Если идёт дождь is the safest and most natural version to use.

It normally means a general habit.

So the idea is:

• Whenever it rains, I take an umbrella.

If you wanted to talk about one specific future event, Russian would usually choose different forms, often with perfective verbs, for example:

• Если пойдёт дождь, я возьму зонт.
• If it starts raining, I’ll take an umbrella.

So your original sentence is best understood as a repeated or typical action, not a one-time future plan.