Od pošty jdeš rovně až na náměstí.

Because the preposition od always requires the genitive case.

The basic noun is pošta = post office / post office building.
In the genitive singular, pošta changes to pošty.

So:

• pošta = the post office
• od pošty = from the post office

This is a very common pattern in Czech:

• od školy = from the school
• od domu = from the house
• od nádraží = from the station

So od pošty simply means from the post office.

Here od means from, specifically from near / from the area of / starting at a place.

In directions, Czech often uses od + genitive to mark the starting point:

• Od pošty... = Starting from the post office...
• Od nádraží... = Starting from the station...

It does not necessarily mean you are physically inside the building and then leaving it. It often just means from that point / from there.

Jdeš is the 2nd person singular present tense of jít = to go / to walk.

Literally it means you go or you are going.

In Czech, directions are often given using the present tense, especially in spoken language:

• Jdeš rovně... = You go straight...
• Pak zahneš doleva. = Then you turn left.

English often uses the imperative in directions, like Go straight, but Czech frequently uses this present-tense instruction style.

A more direct imperative would be:

• Jdi rovně až na náměstí.

That is also possible, but jdeš sounds very natural in route directions.

Yes. Jdeš is 2nd person singular informal, used when speaking to one person you address as ty.

If you want to be polite or if you are speaking to more than one person, you would normally say:

• Od pošty jdete rovně až na náměstí.

So:

• jdeš = informal singular
• jdete = polite singular or plural

Rovně means straight or straight ahead.

It is an adverb, so it describes how you go.

Examples:

• Jdi rovně. = Go straight.
• Jeďte rovně. = Go straight. (when driving / riding)

In this sentence, rovně tells you not to turn.

Here až means something like as far as, all the way to, or up to.

So:

• rovně až na náměstí = straight all the way to the square

Without až, the sentence would still make sense:

• Od pošty jdeš rovně na náměstí.

But až adds the idea of continuing straight until you reach that destination.

It is very common in directions:

• Jdi rovně až ke kostelu. = Go straight as far as the church.
• Jeďte až na konec ulice. = Go all the way to the end of the street.

Czech uses different prepositions with different places, and with náměstí the normal choice is na.

So Czech says:

• na náměstí = to the square / on the square
• na trhu = at the market
• na ulici = in the street / onto the street

Even though English uses to the square, Czech uses na with many open public spaces.

So:

• jít na náměstí = to go to the square

Using do náměstí would sound unnatural in standard Czech.

It is changing case in terms of grammar, but the form happens to look the same.

In this sentence, na expresses movement toward a destination, so it takes the accusative case:

• na náměstí = to the square

The noun náměstí is a neuter noun, and in the singular its nominative and accusative forms are identical.

So although the case is accusative here, the word still looks like náměstí.

This is one reason Czech cases can seem tricky: sometimes the form changes visibly, and sometimes it does not.

You know it from the verb and the context.

The verb jdeš expresses movement, so na náměstí is understood as to the square.

Compare:

• Jdeš na náměstí. = You are going to the square.
• Jsi na náměstí. = You are at the square.

With na, Czech often uses:

• accusative for motion toward a place
• locative for being at a place

But with náměstí, the form is the same in both cases, so the verb helps you interpret it.

Yes, but that is normal in Czech.

The verb ending already shows the subject:

• jdeš = you go

So Czech usually leaves out the pronoun unless it is needed for emphasis or contrast.

You could say:

• Ty jdeš rovně až na náměstí.

But that would usually sound more emphatic, like you go straight...

In neutral directions, the pronoun is normally omitted.

Yes. Czech word order is fairly flexible.

The sentence:

• Od pošty jdeš rovně až na náměstí.

puts od pošty first, which highlights the starting point: from the post office.

You could also hear:

• Jdeš od pošty rovně až na náměstí.

That is also grammatical and natural.

The original version is especially suitable when someone is explaining a route step by step, beginning with the place you start from.

Strictly speaking, jít means to go on foot / to walk.

So jdeš is most natural if the directions are for someone walking.

If the person is driving or riding, Czech often uses jet instead:

• Od pošty jedeš rovně až na náměstí. = From the post office, go straight to the square. / drive straight to the square.

In everyday speech, people may sometimes still use walking-style directions more loosely, but the basic distinction is:

• jít = go on foot
• jet = go by vehicle

Yes, that is a good literal breakdown:

• Od pošty = from the post office
• jdeš = you go
• rovně = straight
• až = all the way / up to
• na náměstí = to the square

A more natural English translation in a directions context would usually be:

• From the post office, go straight to the square.
• From the post office, go straight all the way to the square.

So the literal structure and the natural meaning match quite closely here.