После моста сверните направо у большого здания.

Because после requires the genitive case.

• мост = bridge (dictionary form, nominative)
• after после, it becomes моста

So:

• после моста = after the bridge

This is a very common pattern in Russian:

• после урока = after the lesson
• после работы = after work
• после дома would be grammatically possible, though usually not useful in meaning

Сверните is the imperative form of свернуть, meaning turn or turn off in the sense of changing direction.

In directions, свернуть is very natural for make a turn:

• сверните направо = turn right
• сверните налево = turn left

It is also the formal singular / plural imperative form. So it can mean:

• turn right (when speaking politely to one person)
• turn right (when speaking to more than one person)

The informal singular would be:

• сверни

Because свернуть is not reflexive here.

You only add -ся / -сь when the verb is reflexive. This verb simply means to turn off / to make a turn, so the imperative is:

• сверни — informal singular
• сверните — formal singular or plural

No -ся is needed.

Направо is an adverb, meaning to the right / right.

In directions, Russian often uses adverbs like this:

• направо = right
• налево = left
• прямо = straight ahead

So:

• сверните направо = turn right

You do not need a noun after it. It works as a complete directional word on its own.

Both can mean to the right, and in many situations they are very close.

For basic directions, направо is especially common:

• поверните направо
• сверните направо

Вправо is also correct, but can sound a bit more like toward the right side in movement or orientation.

For a learner, the safest everyday phrase for turn-by-turn directions is:

• сверните направо

Because у in the meaning by / near / next to takes the genitive case.

So:

• здание = building
• у здания = by the building / near the building

The adjective also has to match the noun in case, gender, and number:

• большое здание = a big building
• у большого здания = by the big building

This is why both words change.

Because the adjective must agree with здания, which is in the genitive singular.

Step by step:

• dictionary form: большое здание
  
  • большое = nominative singular neuter
  • здание = nominative singular neuter

After у, both must go into the genitive:

• у большого здания

For neuter singular adjectives in the genitive, -ое usually changes to -ого:

• большое → большого
• новое → нового

It most naturally means by / near the big building.

The preposition у often indicates location close to something:

• у дома = by the house
• у станции = by the station
• у большого здания = by the big building

In directions, this often means the turn happens at the point where the big building is, or next to it, depending on context.

So in English, several translations may work:

• by the big building
• near the big building
• at the big building

In directions, после моста usually means after you pass/cross the bridge.

A native speaker will normally understand:

• first you come to the bridge
• then you go beyond it
• then you turn right

It does not usually mean some vague time much later. In route instructions, it points to the next stage of movement.

Yes, сверните is either:

• formal singular (speaking politely to one person), or
• plural (speaking to multiple people)

That makes the whole sentence sound appropriate for:

• giving directions to a stranger
• GPS/navigation style instructions
• polite public speech

If you were speaking informally to one friend, you would usually say:

• После моста сверни направо у большого здания.

Yes. Russian word order is fairly flexible, although some versions sound more natural than others.

The original:

• После моста сверните направо у большого здания.

This is very natural for directions.

You could also hear:

• После моста у большого здания сверните направо.
• Сверните направо после моста, у большого здания.

These all keep the basic meaning, but the original order is smooth and standard: first the landmark you pass, then the action, then the location marker.

They are very close, and in many direction contexts both are possible.

• поверните направо = turn right
• сверните направо = turn right / turn off to the right

A useful nuance:

• повернуть focuses on turning
• свернуть often suggests leaving the current road/path and turning onto another one

In navigation-style instructions, сверните направо is extremely common.

A helpful pronunciation guide is:

• После моста́ сверни́те напра́во у большо́го зда́ния.

Main stresses:

• после
• моста́
• сверни́те
• напра́во
• большо́го
• зда́ния

A rough English-style approximation:

• POS-lye mas-TA SVYER-nee-tye na-PRA-va oo bal-SHO-va ZDA-ni-ya

Not perfect, but it may help at first.

Because Russian directions often go straight to the key action.

English sometimes says:

• Go past the bridge and turn right...

Russian can simply say:

• После моста сверните направо...

The idea of movement is already understood from the context. This makes the sentence compact and natural.