За мостом начинается тихий парк.

Because за can take different cases depending on the meaning.

Here it means behind / beyond / past the bridge in a stationary location sense, so Russian uses the instrumental case:

• мост → мостом
• за мостом = behind the bridge / beyond the bridge

Compare:

• за мостом = location (behind/beyond the bridge)
• за мост = motion toward a position behind something (to behind the bridge, more naturally to the area beyond the bridge)

So in this sentence, the park is located starting on the far side of the bridge, which is why мостом is instrumental.

In this sentence, за мостом most naturally means beyond the bridge or just past the bridge.

While за often gets taught as behind, with places and landmarks it can also mean on the far side of something:

• за домом = behind the house
• за рекой = beyond the river / across the river
• за мостом = beyond the bridge / past the bridge

So here the idea is that once you go past the bridge, a quiet park begins.

Because the subject is парк, which is:

• masculine
• singular
• nominative

The verb agrees with the subject, not with word order.

So:

• парк начинается = the park begins / starts

Even though парк comes after the verb here, it is still the subject.

The verb is начинаться, which means to begin / to start in an intransitive sense: something itself begins.

So:

• парк начинается = the park begins
• урок начинается = the lesson begins
• фильм начинается = the film starts

By contrast, начинать usually means to begin something:

• Он начинает урок. = He begins the lesson.

The -ся form often gives the meaning that the subject itself is undergoing the action rather than doing it to something else.

Because тихий парк is the subject of the sentence.

The basic structure is:

• парк начинается = the park begins

Since парк is the subject, it is in the nominative singular:

• парк

The adjective must match it:

• тихий = masculine singular nominative
• парк = masculine singular nominative

So:

• тихий парк = a quiet park

Russian word order is flexible, and different orders change emphasis more than basic meaning.

• За мостом начинается тихий парк puts the location first. It feels like: Beyond the bridge, a quiet park begins.
• Тихий парк начинается за мостом puts more focus on the quiet park itself.

Starting with за мостом is very natural if you are describing a scene or guiding someone through a place.

So the original sentence sounds like a descriptive, spatial statement.

You can understand it that way literally, but in natural English that sounds unusual.

A more natural English translation would be something like:

• Beyond the bridge, a quiet park begins.
• Just past the bridge, a quiet park starts.
• A quiet park begins beyond the bridge.

The Russian sentence is perfectly normal, but English usually prefers smoother phrasing.

Because Russian has no articles.

So парк can mean:

• a park
• the park

and the exact meaning depends on context.

Likewise:

• тихий парк can mean a quiet park or the quiet park

In this sentence, English will usually use a quiet park, because the park is being introduced rather than referred to as something already known.

Yes, it is present tense, but like English present tense, it can describe:

1. What is happening now

   • Фильм начинается. = The movie is starting.

2. A general fact or layout

   • За мостом начинается парк. = Beyond the bridge, a park begins.

In your sentence, it is most likely a descriptive/general present: it describes where the park starts geographically.

Yes, absolutely.

• За мостом начинается парк. = Beyond the bridge, a park begins.
• За мостом начинается тихий парк. = Beyond the bridge, a quiet park begins.

The adjective тихий simply adds description. It agrees with парк in:

• gender: masculine
• number: singular
• case: nominative

So it becomes тихий.