В музее мы рассматривали старинную картину.

Because this sentence describes location, not movement.

• в музее = in the museum / at the museum
• в музей = into the museum

After в, Russian uses:

• prepositional case for being somewhere
• accusative case for motion into somewhere

So here, since we were looking at a painting in the museum, the correct form is в музее.

Музей is the dictionary form, called the nominative singular.

After в when it means in/at, the noun goes into the prepositional case:

• музей → в музее

This is a normal pattern for many masculine nouns. The ending changes to show the noun’s role in the sentence.

Because картину is the direct object of the verb рассматривали.

In Russian, direct objects often go into the accusative case.
For a feminine noun ending in -а, the accusative singular usually changes like this:

• картина → картину

So:

• картина = a painting, the painting
• рассматривали картину = were looking at/examining the painting

Because adjectives must agree with the noun they describe in:

• gender
• number
• case

Here the noun is картину, which is:

• feminine
• singular
• accusative

So the adjective must match:

• старинная картина = nominative
• старинную картину = accusative

That is why the adjective changes to старинную.

Not exactly.

• старую means old
• старинную means antique, old-fashioned, or old in a historical/artistic sense

So старинная картина suggests an old, possibly valuable or historical painting, not just a painting that happens to be old or worn out.

Рассматривали means something like:

• were examining
• were looking at carefully
• were studying visually

It is stronger and more specific than just смотрели, which simply means were looking/watching.

So in this sentence, the idea is not just that they glanced at the painting, but that they looked at it with attention.

This is a question of aspect, which is very important in Russian.

• рассматривать / рассматривали = imperfective
• рассмотреть / рассмотрели = perfective

Here рассматривали presents the action as a process or ongoing activity in the past:

• We were examining the painting

If you used рассмотрели, it would sound more like:

• We examined it completely
• We finished looking at it
• We managed to make it out / inspect it fully

So рассматривали fits better when the sentence simply describes what was happening.

In the past tense, Russian verbs agree with the subject in number and, in the singular, also gender.

Because the subject is мы = we, the verb must be plural:

• я рассматривал / рассматривала
• мы рассматривали

So -ли is the normal past plural ending.

It could be omitted.

Russian often leaves out subject pronouns when they are clear from the verb form. So both are possible:

• В музее мы рассматривали старинную картину.
• В музее рассматривали старинную картину.

However, keeping мы can help with:

• emphasis
• contrast
• clarity

In this sentence, мы sounds perfectly natural.

No, Russian word order is fairly flexible.

This sentence begins with В музее to set the scene first:

• In the museum, we were examining an antique painting

But other orders are also possible, for example:

• Мы рассматривали старинную картину в музее.
• Старинную картину мы рассматривали в музее.

The basic meaning stays the same, but the focus or emphasis changes.

Russian has no articles like a or the.

So картину by itself can mean:

• a painting
• the painting

The exact choice in English depends on context. Russian speakers rely on context, word order, and shared knowledge instead of articles.

The main stress pattern is:

В музе́е мы рассма́тривали стари́нную карти́ну.

Word by word:

• музе́е
• рассма́тривали
• стари́нную
• карти́ну

Learning stress is important in Russian because it is not always predictable from spelling.