Když nemám čas stát na poště ve frontě, dám dopis do schránky.

Both are possible in English, but když basically means when.

In sentences like this, Czech often uses když for a situation that happens repeatedly or whenever the condition comes up. So the sense is:

• When/whenever I don’t have time...
• and in natural English that can also sound like If I don’t have time...

If you want a more clearly conditional if, Czech often uses jestli or pokud. But když is very common in everyday Czech for this kind of real-life condition.

Because that whole part is a subordinate clause introduced by když.

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

• Když nemám čas stát na poště ve frontě, dám dopis do schránky.

This is true even when English might be less strict in informal writing.

Because Czech commonly uses mít čas + infinitive to mean to have time to do something.

So:

• mám čas počkat = I have time to wait
• nemám čas psát = I don’t have time to write
• nemám čas stát = I don’t have time to stand

Here stát is the action the speaker does not have time for.

Yes, stát literally means to stand, but stát ve frontě is the normal Czech expression for to stand in line / queue.

So this is an idiom:

• stát ve frontě = to stand in a queue
• natural English: to wait in line

Even if in real life you might not literally be standing the whole time, Czech still uses stát ve frontě.

Because Czech prepositions are often idiomatic, and different nouns naturally go with different ones.

Here:

• na poště = at the post office
• ve frontě = in the queue

So:

• na
  • location of an institution/place: na poště
• v / ve
  • being inside a line/queue: ve frontě

You usually just have to learn these combinations as standard expressions:

• být na poště = to be at the post office
• stát ve frontě = to stand in line

Because ve is a pronunciation-friendly form of v.

Czech often uses ve instead of v when the next word begins with a consonant cluster or with a sound that would make pronunciation awkward. With frontě, ve frontě sounds smoother than v frontě.

This happens in other common phrases too:

• ve škole
• ve městě
• ve středu

So this is mostly about ease of pronunciation.

Because na poště expresses location, while na poštu expresses motion toward the place.

Compare:

• jdu na poštu = I’m going to the post office
• jsem na poště = I’m at the post office

In your sentence, the speaker is talking about standing in line at the post office, so it must be na poště.

Because dám is from the perfective verb dát, which focuses on a completed action: I’ll put / I’ll drop.

Here the idea is:

• if that situation happens, then I perform one complete action:
• I put the letter into the mailbox

So dám works very naturally.

A useful thing to know: the present tense of a perfective verb often refers to the future in English.

So:

• dám = I will put / I’ll give

By contrast, dávám is imperfective and means something more like:

• I am putting
• I usually put
• I keep putting

In this sentence, dám sounds more like a single concrete result.

Because do means movement into something, and it takes the genitive case.

So:

• schránka = mailbox / box
• do schránky = into the mailbox

This is a very common pattern:

• do školy = into/to school
• do domu = into the house
• do auta = into the car

Here schránky is the genitive singular form of schránka.

Also, in this postal context, schránka naturally means a mailbox / postbox.

It is in the accusative. It just happens to look the same as the nominative.

Dopis is a masculine inanimate noun, and for many masculine inanimate nouns, the singular nominative and accusative are identical:

• dopis = letter
• mám dopis = I have a letter
• dám dopis do schránky = I put the letter into the mailbox

So the case is accusative here, even though the form does not change.

A quick breakdown:

• čas — accusative, object of nemám
• stát — infinitive
• na poště — locative after na for location
• ve frontě — locative after ve
• dopis — accusative, direct object of dám
• do schránky — genitive after do

So the sentence is a nice example of how Czech prepositions control case.

Yes. Czech word order is fairly flexible, and stát ve frontě na poště is completely natural.

In fact, many learners may find this version easier to process because ve frontě stays closer to stát, forming the common phrase stát ve frontě.

So both are understandable:

• stát na poště ve frontě
• stát ve frontě na poště

The difference is mainly rhythm and emphasis, not basic meaning.