Learn Hardcore Czech: Důvod, proč chodím nejčastěji do knihovny, je ten, že je tam ticho. - The reason why I most often go to the library is that it is quiet there.

So here proč introduces a relative clause (like why after reason) rather than a direct question word. Czech can do this the same way English does: důvod, proč… = the reason why…

Why is there a comma before proč in Důvod, proč chodím…?

The comma separates the main noun důvod (reason) from the relative clause proč chodím nejčastěji do knihovny which describes that noun.

Think of it like English:

The reason, why I go to the library most often, is that…

In natural English you’d usually drop that comma, but in Czech a comma is standard before a relative clause introduced by který, kde, kdy, proč etc. when they expand on a noun.

Why is it chodím and not jdu or jdu do knihovny?

Chodím is the imperfective verb in the habitual sense: it means I go (regularly / usually).

chodím do knihovny = I (tend to) go to the library / I often go there (repeated action)
jdu do knihovny = I am going to the library (now, this time – one specific event)

Because the sentence talks about what usually happens (nejčastěji – most often), chodím is the correct choice.

What exactly does nejčastěji mean and how is it formed?

často = often
častěji = more often (comparative)
nejčastěji = most often (superlative)

So nejčastěji literally means “the most often” / “most frequently”.

Formally:

base adverb: často
comparative: častěji
add nej- to the comparative: nejčastěji

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

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

knihovna – nominative (dictionary form)
knihovny – genitive singular

So do + GEN → do knihovny.
You cannot say do knihovnu; that would be mixing preposition and the wrong case ending.

What does ten do in je ten, že je tam ticho? Can I leave it out?

ten here is a demonstrative pronoun that works like a kind of “placeholder” or “link”:

Důvod … je ten, že…
= The reason … is (the following), that …

It makes the structure more explicit and a bit more formal or careful in style.

You can omit it:

Důvod, proč chodím nejčastěji do knihovny, je, že je tam ticho.

This is also correct and maybe a bit more neutral. With ten, the sentence feels slightly more formal or emphasized, like “the reason is this: …”

Why is there a comma before že?

že introduces a subordinate clause (a that-clause):

… je ten, že je tam ticho.
= … is that there is silence there.

In Czech, a comma normally separates the main clause from a subordinate clause introduced by conjunctions like že, protože, když, aby etc. So the comma before že is mandatory here.

Could I say že tam je ticho instead of že je tam ticho?

Yes, both že je tam ticho and že tam je ticho are grammatically fine.

že je tam ticho – more neutral order.
že tam je ticho – slightly stronger stress on tam (that there, in that place, it is quiet).

The difference is subtle, and both are very common. In this sentence, že je tam ticho is probably the most natural neutral version.

Why is the subject já (I) missing before chodím?

In Czech, the verb ending usually shows the person and number clearly, so subject pronouns are often dropped when they’re obvious.

chodím already means I go (1st person singular).
Adding já would emphasize the subject: Já chodím nejčastěji do knihovny… = I (as opposed to someone else) go most often to the library…

So the natural, unmarked sentence just uses chodím without já.

What is ticho exactly – a noun or an adjective? Why not tichý?

ticho is a noun meaning silence or quiet.

je tam ticho literally = there is silence there / it is quiet there
(literally: there is silence there)

tichý is an adjective meaning quiet:

tichá knihovna = a quiet library

In expressions like je tam ticho, Czech prefers the noun. English often uses an adjective (“it’s quiet there”), but Czech very naturally says (je) ticho (“(there is) silence”).

Could I say Nejčastěji chodím do knihovny, protože je tam ticho instead? Is there any difference?

Yes, that’s a perfectly good alternative:

Nejčastěji chodím do knihovny, protože je tam ticho.
= I go to the library most often because it’s quiet there.

Differences:

Důvod, proč … je ten, že … – more explicit, a bit more formal or written style: “The reason why … is that …”.
… protože … – simpler, more conversational: “because …”.

Both express basically the same content, just with different style and structure.

Is the word order chodím nejčastěji do knihovny fixed, or could I say chodím do knihovny nejčastěji?

You can say both:

chodím nejčastěji do knihovny
chodím do knihovny nejčastěji

Both are correct. Word order in Czech is fairly flexible and is used to adjust emphasis. Roughly:

chodím nejčastěji do knihovny – slight emphasis on how often relative to other places (the most often place is the library).
chodím do knihovny nejčastěji – slight emphasis on how often I go there (to the library I go the most often).

The difference is small; both sound natural.