V knihovně je moc lidí.

Because after the preposition v when it means in, Czech uses the locative case.

• knihovna = the basic dictionary form, library
• v knihovně = in the library

So the ending changes because the noun is not standing alone; it is being used after v in a location phrase.

Czech sometimes changes v to ve to make pronunciation easier, especially before difficult consonant clusters.

But with knihovně, normal Czech uses v knihovně.

Compare:

• v knihovně
• ve škole
• ve městě
• ve vlaku

So ve is not automatic; it depends on what sounds natural in Czech.

Because moc lidí is treated as a quantity expression: literally something like a lot of people / many people as one amount.

In Czech, with expressions such as:

• moc lidí
• hodně lidí
• málo lidí
• pět lidí

the verb is very often singular in this kind of existential sentence.

So:

• V knihovně je moc lidí. = normal
• Lidé jsou v knihovně. = different structure, where lidé is the clear plural subject

In other words, je here matches the whole idea of there is a large number of people, not the individual people one by one.

Because after words of quantity like moc, Czech normally uses the genitive plural.

• lidé = nominative plural, people
• lidí = genitive plural, used after quantity words

So:

• lidé jsou... = people are...
• moc lidí = a lot of people

You cannot normally say moc lidé.

The full pattern is a little irregular:

• singular: člověk = person
• plural nominative: lidé = people
• plural genitive: lidí

So in this sentence, lidí belongs to the noun whose basic singular form is člověk.

This is one of the irregular common nouns that learners simply need to get used to.

It can mean either, depending on context and tone.

In everyday Czech:

• moc lidí can mean a lot of people
• in some situations it can also feel like too many people

If you want to be very clear:

• hodně lidí = a lot of people
• mnoho lidí = many / a lot of people
• příliš mnoho lidí = too many people

So moc is common and natural, but it can be slightly context-dependent.

Yes, it is very natural.

Moc lidí is common in everyday spoken Czech. It sounds normal and idiomatic.

Some alternatives:

• hodně lidí = very common, everyday
• mnoho lidí = a bit more formal or written
• příliš mnoho lidí = clearly too many people

So if you hear V knihovně je moc lidí, that sounds perfectly normal in conversation.

Because Czech does not need a dummy subject like English there.

English says:

• There are a lot of people in the library.

Czech simply says:

• V knihovně je moc lidí.
• literally something like In the library is a lot of people

So Czech uses the verb directly and does not insert an empty word like English there.

Yes. Czech word order is much more flexible than English word order.

The version V knihovně je moc lidí is natural because it starts with the place, then gives the information about what is there.

Other possible orders include:

• Je v knihovně moc lidí.
• Moc lidí je v knihovně.

But the emphasis changes:

• V knihovně je moc lidí = focuses first on the location
• Moc lidí je v knihovně = focuses more on a lot of people

So the original sentence is a very natural way to say it, especially if the library is already the topic.

Because Czech has no articles like English a/an/the.

So knihovna can mean:

• a library
• the library

The context tells you which one is meant.

In V knihovně je moc lidí, English often uses the library, but Czech does not need a separate word for that.

A rough pronunciation guide is:

f KNI-ho-vnye ye mots LEE-dee

A few important points:

• v before k is pronounced like f
• je sounds like ye
• moc sounds like mots
• lidí has a long final í
• ň in knihovně is a soft n sound, somewhat like ny

Also, Czech stress usually falls on the first syllable of the word or phrase, so:

• v knihovně is stressed on kni-
• lidí is stressed on li- even though the last vowel is long