Learn Hardcore Czech: Když prší, beru si s sebou deštník i do knihovny. - When it rains, I take an umbrella with me even to the library.

pršet = “to rain” is an impersonal verb.
You just say Prší. = “It’s raining.” (literally “Rains.”)
So Když prší literally is “When rains”, but it’s the normal way to say “When it rains”.

You almost never add a subject like to (“it”) here; To prší sounds wrong in standard Czech.

Is Když prší, beru si s sebou deštník i do knihovny. talking about a habit, the present, or the future?

This sentence describes a general habit:

Když prší – “When it rains” (whenever that happens)
beru si – present tense of an imperfective verb, used here for something repeated or usual.

So the natural English sense is:
“When it rains, I (usually) take an umbrella with me, even to the library.”

For one specific future situation, you’d normally change the verbs (see next question).

How would I say “When it rains, I will take an umbrella to the library” for one specific future situation?

You’d normally use a future time clause and a perfective verb in the main clause:

Když bude pršet, vezmu si s sebou deštník do knihovny.
= “When it rains / when it is raining, I will take an umbrella with me to the library.”

Structure:

bude pršet – future of “to rain”
vezmu si – perfective (vzít si) = “I’ll take (once, as a single act)”

Why is it beru si and not just beru?

The verb here is really the reflexive verb brát si:

brát = “to take, to be taking (in general)”
brát si něco = “to take something for oneself / to take something with oneself”

So:

Beru deštník. – “I take the umbrella.” (neutral, can be OK e.g. in some contexts)
Beru si deštník. – “I take an umbrella (for myself/to have it with me).”

In the meaning “I take an umbrella with me”, brát si (with si) is the usual, natural verb.

What exactly does s sebou add here? Isn’t si already “to myself / with me”?

They do two different jobs:

si belongs to the verb brát si = “to take for oneself / take along”.
s sebou literally = “with oneself”.

The phrase brát si něco s sebou is a very common fixed combination meaning:

“to take something (along) with you”

So:

Beru si deštník. – “I (habitually) take an umbrella (for myself).”
Beru si s sebou deštník. – “I (habitually) take an umbrella with me.”

In many contexts s sebou can be omitted without changing the meaning much, but including it makes the “with me” part explicit.

What does i mean in deštník i do knihovny, and why is it placed there?

i here means “even / also”, and its position shows what is being emphasized.

In … beru si s sebou deštník i do knihovny, the focus is:

“I take an umbrella with me, even to the library.”

That suggests the library is a place where you might normally not bother with an umbrella, but even there you take it.

Compare:

… beru si s sebou i deštník do knihovny.
→ “I even take an umbrella with me to the library.”
(emphasis more on umbrella as an extra thing you also take)
… beru si s sebou deštník i do školy, i do knihovny.
→ “I take an umbrella with me both to school and to the library.”

So i attaches to what follows it and highlights that element (“even/too/as well”).

Why is it do knihovny and not something like do knihovna?

The preposition do (“to, into”) requires the genitive case.

The noun knihovna (“library”) is a regular feminine -a noun:

Singular:

nominative (dictionary form): knihovna
genitive: knihovny

After do you need genitive, so:

do knihovny = “to the library”

do knihovna is ungrammatical; the ending must change to show the case.

What case is knihovny here, and why do we use that case with do?

Here knihovny is genitive singular of knihovna.

Preposition do (meaning physical or abstract “into / to”) is always followed by genitive.
The genitive often appears after prepositions of movement into / towards a place.

So:

jdu do knihovny – “I’m going to the library.”
jdu do školy – “I’m going to school.”
jdu do práce – “I’m going to work.”

All those nouns are in the genitive after do.

What’s the difference between do knihovny and v knihovně?

They differ in both preposition and case, and therefore in meaning:

do knihovny – “to the library” (movement towards/into the library)
- do
  - genitive (knihovny)
v knihovně – “in the library” (location inside the library)
- v
  - locative (knihovně)

So:

Beru si s sebou deštník do knihovny.
= I take the umbrella to the library (when I go there).
Mám s sebou deštník v knihovně.
= I have the umbrella in the library (while I’m there).

Why is there a comma after Když prší?

Because když prší is a subordinate clause (“when it rains”), and Czech spelling rules require a comma between a subordinate clause and the main clause.

Pattern:

[Subordinate clause], [main clause].

So:

Když prší, beru si s sebou deštník i do knihovny.
Když mám čas, chodím do knihovny. – “When I have time, I go to the library.”

If the order is reversed, you still write the comma:

Beru si s sebou deštník i do knihovny, když prší.

Could I say something like Při dešti beru s sebou deštník instead of Když prší? Does it sound natural?

You can say it, but it sounds more formal or bookish:

Při dešti beru s sebou deštník.
literally “During rain I take an umbrella with me.”

Nuances:

Když prší – the normal, everyday way: “when it rains”.
Při dešti – “in case of rain / during rain”; more formal, often used in written instructions, notices, or more literary style.

In casual speech about your habits, Když prší, beru si s sebou deštník… is much more natural.

Is deštník the standard word for “umbrella”? Anything important to know about it?

Yes, deštník is the normal everyday word for a rain umbrella.

A few basics:

Gender: masculine inanimate.
- sg: deštník, deštníku, deštníku, deštník, dešníku, deštníkem
- pl: deštníky, deštníků, deštníkům, deštníky, deštnících, deštníky
Example:
- Mám deštník. – I have an umbrella.
- Bez deštníku nejdu. – I’m not going without an umbrella.

There is also paraple, a bit old‑fashioned/colloquial and sometimes humorous, but deštník is the safe, neutral choice.