Vedle pošty je červená schránka, do které lidé dávají dopisy.

Because vedle is a preposition that requires the genitive case when it means next to / beside.

• dictionary form: pošta = post office
• genitive singular: pošty

So:

• vedle pošty = next to the post office

This is very common in Czech: many prepositions force the following noun into a particular case.

Here vedle means next to, beside, or alongside.

In this sentence it describes location:

• Vedle pošty = beside the post office

It is one of several Czech location expressions you will see often, for example:

• u pošty = by/near the post office
• před poštou = in front of the post office
• za poštou = behind the post office

So vedle is specifically about being at the side of something.

Czech does not need a separate dummy word like English there in there is / there are.

So:

• Vedle pošty je červená schránka.

literally looks like:

• Beside the post office is a red mailbox.

But in natural English we usually say:

• There is a red mailbox beside the post office.

So je is simply is, but in this kind of sentence it often functions like English there is.

Because adjectives in Czech must agree with the noun they describe in:

• gender
• number
• case

The noun schránka is:

• feminine
• singular
• nominative here

So the adjective červený changes to the matching form:

• masculine: červený
• feminine: červená
• neuter: červené

That is why you get:

• červená schránka = red mailbox

Because schránka is the subject of the clause, and subjects are normally in the nominative case.

In:

• Vedle pošty je červená schránka

the thing that is there is červená schránka, so it stays in nominative:

• schránka

If it were a direct object, then you might see schránku instead.

Yes, in this context schránka means a mailbox / post box.

The word schránka can also mean a box, container, or even things like an inbox in other contexts. Czech often uses broad words whose exact meaning depends on the situation.

Because the sentence mentions:

• pošta = post office
• dopisy = letters

it is clear that schránka here is a postal box.

Do které means into which.

It is made of:

• do = into
• které = which / that

This starts a relative clause describing schránka:

• červená schránka, do které lidé dávají dopisy
• a red mailbox into which people put letters

In smoother English, you would often say:

• a red mailbox that people put letters into

So Czech uses a relative pronoun where English may leave the preposition to the end.

Because the form of který is determined by two things at once:

1. it agrees with the noun it refers to
   
   • here it refers to schránka, which is feminine singular
2. its case depends on its role in the relative clause
   
   • here it follows do, and do requires the genitive

So we need the feminine singular genitive form of který:

• nominative: která
• accusative: kterou
• genitive: které

Since the phrase is do které, the correct form is které.

Because do které lidé dávají dopisy is a subordinate relative clause, and Czech normally separates subordinate clauses with commas.

So the comma is standard punctuation here:

• Vedle pošty je červená schránka, do které lidé dávají dopisy.

English punctuation is a bit less automatic in similar structures, but in Czech this comma is expected.

Lidé is the standard nominative plural form meaning people.

It is related to člověk = person, but the plural system is irregular:

• singular: člověk
• plural: lidé

You may also hear lidi in spoken Czech, and it is very common in everyday speech, but lidé is the more standard form, especially in careful written language.

In this sentence, lidé is the subject of dávají.

Because the subject lidé is third person plural, so the verb must agree with it.

The verb is from dávat = to give / put (imperfective), and the present tense form is:

• já dávám
• ty dáváš
• on/ona dává
• my dáváme
• vy dáváte
• oni dávají

So:

• lidé dávají = people put / give

This is a question of aspect, which is very important in Czech.

• dávat = imperfective
• dát = perfective

Here the sentence describes a habitual, general action:

• people put letters into the box
• this happens regularly, not just once

So Czech uses the imperfective verb:

• dávají

If you used dají, it would sound more like a completed act or a one-time event:

• they will put / they put and complete the action

For a general truth or repeated action, dávají is the natural choice.

Because dopisy is the direct object of dávají, so it is in the accusative plural.

• singular: dopis = letter
• plural nominative: dopisy
• plural accusative: dopisy

For masculine inanimate nouns like dopis, the plural nominative and plural accusative are often the same, which is why the form does not change visibly here.

So:

• lidé dávají dopisy = people put letters