Od mostu musíš jít ještě rovně až na poštu.

Od means from / away from, and it takes the genitive case.

• dictionary form: most = bridge
• genitive singular: mostu

So:

• od mostu = from the bridge
• literally, it marks the starting point of the route

This is very common in directions:

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

Yes. Musíš is the 2nd person singular form of muset = must / have to.

So musíš means you must or you have to.

It is informal singular, used when speaking to one person you address as ty.

Compare:

• musíš = you must (informal, one person)
• musíte = you must (formal, or plural)

In directions, musíš often sounds a bit like you need to rather than a strong command.

Because Czech often leaves out subject pronouns when they are obvious from the verb ending.

• musíš already tells you the subject is you singular

So Czech does not need ty here.

You could say Ty musíš..., but that usually adds emphasis, contrast, or emotion.

Jít means to go on foot or to walk/go in one direction.

That fits directions very well:

• musíš jít rovně = you have to go straight

This is different from:

• jet = go by vehicle
• chodit = go/walk habitually or repeatedly

So here jít is the natural choice because it describes one specific route.

Here ještě means something like:

• still
• a bit more
• further
• continue

So jít ještě rovně means keep going straight a little further / continue straight.

This is a very common use in directions. It suggests that the person should not turn yet.

Rovně is an adverb meaning:

• straight
• straight ahead

In directions, it is extremely common:

• Jdi rovně. = Go straight.
• Pořád rovně. = Keep going straight.
• Jeď rovně. = Go straight (by vehicle).

It does not change form here, because it is an adverb, not an adjective.

In this sentence, až means as far as / all the way to / up to.

So:

• až na poštu = all the way to the post office

It marks the endpoint.

This is a very common use in directions:

• Jeď až ke kostelu. = Go all the way to the church.
• Jdi až na konec ulice. = Go all the way to the end of the street.

So here až tells you not to stop before the post office.

In Czech, movement to certain public places or institutions is often expressed with na + accusative.

So Czech commonly says:

• na poštu = to the post office
• na nádraží = to the station
• na policii = to the police station
• na univerzitu = to university

This is just the normal Czech pattern for many destinations.

Here:

• pošta = post office
• na poštu = to the post office

A learner often wants to use do because English says to, but Czech does not always match English preposition-for-preposition.

Poštu is the accusative singular of pošta.

Why accusative? Because na with a destination of movement usually takes the accusative.

• pošta = nominative
• na poštu = accusative after motion toward a place

Compare:

• Jdu na poštu. = I’m going to the post office.
• Jsem na poště. = I’m at the post office.

Notice the difference:

• na poštu = motion toward
• na poště = location

Czech word order is flexible, and the beginning of the sentence often gives the starting point or topic first.

So Od mostu musíš jít ještě rovně až na poštu naturally starts with the place you begin from: from the bridge.

That is very normal in directions, because it frames the route right away.

A different order is possible, for example:

• Musíš jít od mostu ještě rovně až na poštu.

But the original version sounds very natural because it begins with the route starting point.

Yes, it strongly suggests walking directions because of jít.

If the speaker wanted to talk about going by car, bus, bike, etc., Czech would more likely use another verb, especially jet.

Compare:

• musíš jít = you have to go / walk
• musíš jet = you have to go by vehicle

So the sentence is most naturally understood as telling someone how to continue on foot from the bridge to the post office.