Když bude pršet, vezmi si s sebou deštník.

Because this sentence talks about a future situation.

• pršet = to rain
• bude pršet = it will rain / it will be raining

In Czech, weather verbs like pršet form the future with bude + infinitive.

Also, unlike English, Czech can use a real future form after když:

• Když bude pršet... = When/If it rains...

English usually says when it rains, not when it will rain, but Czech normally does use the future here.

Když usually means when, but in some contexts it can feel close to if.

In this sentence, it introduces a condition connected with time:

• Když bude pršet, ... = When it rains / If it rains, ...

So a learner should understand that když is often used very naturally in sentences where English might choose either when or if, depending on context.

Vezmi is the singular imperative of vzít (to take).

So it is used when speaking to one person informally:

• vezmi = take

Related forms:

• vezměte = plural or polite singular imperative (take)
• vezmi si = take / take for yourself

The verb vzít is perfective, which fits well here because the speaker means do this one complete action.

Si is a short reflexive/dative word that often appears in Czech with actions done for oneself.

So:

• vezmi deštník = take an umbrella
• vezmi si deštník = more naturally take an umbrella for yourself / take an umbrella

In many everyday Czech sentences, si makes the expression sound more natural and idiomatic, even when English would not translate it directly.

Here it suggests something like: make sure you take one with you for your own use.

S sebou means with you / along with you.

So:

• vezmi si s sebou deštník = take an umbrella with you

This phrase adds the idea of bringing something along.

You can compare:

• Vezmi si deštník. = Take an umbrella.
• Vezmi si s sebou deštník. = Take an umbrella with you.

The second one is more explicit.

Because the person who is taking the umbrella and the person the umbrella goes with are the same person.

Czech uses the reflexive form sebou in that situation:

• s tebou = with you (with another person you are talking to)
• s sebou = with oneself / with you yourself in a reflexive meaning

So in vezmi si s sebou deštník, the idea is take the umbrella along with yourself.

Deštník is the direct object of vzít, so it is in the accusative case.

The noun deštník is masculine inanimate, and in the singular its accusative has the same form as the nominative:

• nominative: deštník
• accusative: deštník

So even though it is accusative here, the word looks unchanged.

Because Když bude pršet is a subordinate clause.

Czech normally separates a subordinate clause from the main clause with a comma:

• Když bude pršet, vezmi si s sebou deštník.

This is standard Czech punctuation. English also often uses a comma when the when/if clause comes first, so this may feel familiar.

Yes, Czech word order is fairly flexible, but not completely free.

This sentence could also be said as:

• Vezmi si s sebou deštník, když bude pršet.

The original version puts the condition first:

• Když bude pršet, ...

That sounds natural because it sets up the situation before giving the instruction.

Also notice that si usually appears in the second position of its clause, which is why vezmi si is the normal order.

Yes, sometimes you can, but the nuance is a little different.

• když bude pršet = when it rains / if it rains
• jestli bude pršet = more clearly if it rains / whether it rains

In everyday speech, když is very common in this kind of practical advice. It can sound more natural when the speaker is talking about a likely future situation.

So Když bude pršet, vezmi si s sebou deštník is a very normal Czech sentence.