Мы пошли пешком по широкому проспекту, а потом свернули на тихий бульвар.

Пошли is the past tense of пойти, a perfective verb. Here it means they set off / started going / went as a completed action.

Шли is the past tense of идти, an imperfective verb. It would focus more on the process: they were walking / they went along.

So in this sentence, Мы пошли пешком... suggests a clear next step in the story: we set off on foot.

You're right that идти / пойти is a verb of motion normally associated with movement on foot. But пешком is still often added for clarity or emphasis.

It can suggest:

• specifically on foot
• rather than by car, bus, metro, etc.
• a more natural, explicit way to describe the manner of travel

So пошли пешком is very natural and means something like we went on foot / we decided to walk.

Пешком is an old instrumental-form adverbial expression meaning on foot.

For learners, the easiest thing is to memorize it as a fixed expression:

• идти пешком = to go on foot
• пойти пешком = to set off on foot

You do not normally change it in this use.

Because the preposition по here means along / down and requires the dative case.

So:

• широкий проспект = nominative
• по широкому проспекту = dative

That is why both words change:

• широкий → широкому
• проспект → проспекту

The phrase means along the wide avenue.

In this sentence, по means along or down a street or route.

So:

• идти по улице = to walk along the street
• ехать по дороге = to drive along the road
• по широкому проспекту = along the wide avenue

It does not mean on in the English physical-contact sense here. It is about movement along a path.

After verbs like свернуть when you mean turn onto a street, Russian uses на + accusative.

So:

• тихий бульвар = accusative singular masculine
• for an inanimate masculine noun, the accusative looks the same as the nominative

That is why you see:

• на тихий бульвар = onto the quiet boulevard

This is different from по широкому проспекту, which describes movement along a place, not movement onto it.

Because they are in different cases.

• по широкому проспекту uses the dative
• на тихий бульвар uses the accusative

For masculine singular inanimate nouns:

• the accusative often looks like the nominative
• so тихий бульвар stays тихий бульвар

But the dative changes:

• широкий проспект → широкому проспекту

So the difference comes from the grammar required by the prepositions по and на.

In this sentence, а потом means something like and then / and after that, but with a slight sense of moving on to the next stage.

Russian а often links two parts of a story with a mild contrast or transition:

• first we did one thing,
• then we did another.

So:

• ..., а потом свернули... = ..., and then we turned...

Using и потом would also be understandable in many contexts, but а потом sounds very natural for narration.

Свернули is the past plural form of свернуть, a perfective verb meaning:

• turned
• turned off
• turned onto

In this sentence, it means:

• we turned onto a quiet boulevard

Because it is perfective, it presents the turn as a completed event in the sequence of actions.

Because the subject is мы = we.

In the Russian past tense, verbs agree with the subject in gender/number:

• я пошёл / пошла
• он пошёл
• она пошла
• мы пошли
• мы свернули

So пошли and свернули are the correct plural forms for we.

Russian word order is more flexible than English, although the original sentence is the most neutral and natural.

Original:

• Мы пошли пешком по широкому проспекту, а потом свернули на тихий бульвар.

You could also say, for example:

• Потом мы свернули на тихий бульвар.
• По широкому проспекту мы пошли пешком...

But changing the order usually changes the focus or emphasis, not the core meaning.

The given version sounds like normal storytelling:

1. we went along the avenue
2. then we turned onto the boulevard

Because а is a coordinating conjunction joining two clauses:

• Мы пошли пешком по широкому проспекту
• а потом свернули на тихий бульвар

In Russian, clauses joined by а are normally separated by a comma.

So the comma is standard punctuation here.

These are two different kinds of streets.

• проспект usually means a major avenue, often broad and important
• бульвар usually means a boulevard, often calmer or more scenic, sometimes with trees or a central walkway

So the sentence paints a clear image:

• first, movement along a wide avenue
• then, a turn onto a quiet boulevard