Learn Hardcore Russian: Когда мы гуляли по центру, гид показал нам главную достопримечательность города. - When we were walking in the center, the guide showed us the main landmark of the city.

Because the sentence starts with a subordinate clause introduced by когда (When…). In Russian, a когда-clause is normally separated from the main clause by a comma:

Когда мы гуляли по центру, … = When we were walking around the center, …

What tense is гуляли and why does it look like “we were walking” rather than “we walked”?

Гуляли is past tense plural of гулять. The verb is imperfective, which often describes an ongoing/background action in the past (“were walking,” “used to walk,” “walked around” depending on context). Here it sets the scene while the main event happens.

Why is показал used instead of показывал?

Показал is perfective past (one completed action): the guide showed (successfully, as a single event).
Показывал would be imperfective and would suggest a process/repetition (“was showing,” “used to show,” “showed things for a while”), which doesn’t fit as neatly with one specific highlight.

What case is по центру, and why is it not в центре?

По центру uses по + dative: центр → центру. With movement, по often means “around/throughout/along (an area)”:

гулять по центру = walk around the city center (moving within it)

В центре is usually “in the center” as a location (more static):

гулять в центре is possible, but it can feel more like “walk while being located in the center,” less “covering the area.”

Why is центру not центре? I thought “in the center” is в центре (prepositional).

Because по requires the dative in this meaning: по центру.
в центре uses в + prepositional, but that’s a different preposition and slightly different nuance.

Why is it гид показал нам, not гид показал мы or гид показал нас?

Because нам is dative (“to us”). With показать, Russian commonly marks the person who is shown something with the dative:

показал кому? нам = showed to us The thing being shown takes accusative (see the next question).

What case is главную достопримечательность, and why?

It’s accusative because it’s the direct object of показал (showed what?):

показал (что?) главную достопримечательность Both words are feminine singular accusative:
главная → главную
достопримечательность stays the same in nominative/accusative because it’s a soft-sign feminine noun.

Why is it достопримечательность города (genitive), not достопримечательность город or города достопримечательность?

Города is genitive meaning “of the city”:

достопримечательность (чего?) города = a landmark of the city Word order can vary, but the neutral, most common order is exactly what you see: noun + genitive dependent (достопримечательность города). Putting города first is possible but more stylistically marked.

Does главную mean “main” as in “principal,” or “major” as in “biggest”?

Главную means “main/primary/most important.” It doesn’t strictly mean “biggest.” In travel contexts it often means “the top/number-one landmark.”

Could I say Когда мы гуляли в центре instead, and would it change the meaning?

Yes, it’s grammatical. The difference is nuance:

гуляли по центру = walking around the center area (covering it, moving through it)
гуляли в центре = walking while in the center (location-focused, slightly less “around”)

Why is мы included? Could it be omitted?

Russian often drops subject pronouns because the verb ending already shows the person/number. You could say:

Когда гуляли по центру, гид показал нам… But including мы makes it clearer and a bit more explicit/neutral, especially for learners or when context might be ambiguous.

What is the most natural word stress/pronunciation to watch out for?

Common stress points here:

когдА
мы гулЯли
по цЕнтру
гид показАл
нам
глАвную достопримечАтельность
гОрода