Друг перевернул картину и разгадал головоломку.

Друг is the subject of the sentence, so it is in the nominative case. Its basic dictionary form is also друг, meaning friend.

A couple of useful notes:

• It is a masculine singular noun.
• Russian has no articles, so друг can mean a friend or the friend, depending on context.

Because it is past tense.

In Russian, past tense verbs are formed from the infinitive stem plus endings that show gender and number:

• masculine: -л
• feminine: -ла
• neuter: -ло
• plural: -ли

So:

• перевернул = he turned over / a male subject turned over
• перевернула = she turned over
• перевернули = they turned over

Since друг is masculine singular, перевернул matches it.

The infinitive is перевернуть.

This is a perfective verb, which means it presents the action as completed: the friend turned the picture over and finished doing so.

A common imperfective partner is переворачивать or in past tense переворачивал, which would focus more on the process, repeated action, or ongoing action.

So here:

• перевернул = completed action
• переворачивал = was turning over / used to turn over / turned over repeatedly

Because it is the direct object of the verb перевернул, so it must be in the accusative case.

Картина is a feminine noun ending in -а. For many feminine nouns like this, the accusative singular changes:

• картина → картину

So:

• nominative: картина
• accusative: картину

This is very common with feminine nouns.

For the same reason: головоломка is the direct object of разгадал, so it is in the accusative singular.

The pattern is:

• головоломка → головоломку

So both objects in the sentence are feminine singular nouns in the accusative:

• картину
• головоломку

Разгадал is also perfective, from the infinitive разгадать.

That means the puzzle was successfully solved. The action is presented as reaching its result.

Compare:

• разгадал = solved, figured out
• разгадывал = was solving / tried to solve / solved in a process-oriented sense

In this sentence, both actions are complete:

• he turned the picture over
• he solved the puzzle

So perfective verbs fit very naturally.

Because Russian normally does not use articles.

So:

• друг can mean a friend or the friend
• картину can mean a picture/the picture
• головоломку can mean a puzzle/the puzzle

Context tells you which is meant. English requires articles; Russian usually does not.

Yes. И is the basic coordinating conjunction meaning and.

Here it links two verbs with the same subject:

• Друг перевернул картину
• (Друг) разгадал головоломку

Russian does not need to repeat друг in the second part because it is already clear that the same person did both actions.

Yes. Russian word order is fairly flexible because case endings show grammatical roles.

The sentence as given is a neutral, natural order:

• Друг перевернул картину и разгадал головоломку.

But other orders are possible for emphasis, for example:

• Картину друг перевернул и разгадал головоломку.
• Головоломку друг разгадал, перевернув картину.

These alternatives may sound more marked or emphasize different parts. The original sentence is the most straightforward way to say it.

It can mean either, depending on context.

Картина often means:

• painting
• picture
• sometimes even scene or overall image in other contexts

In this sentence, the exact English choice depends on the situation. Grammatically, though, nothing changes.

You would normally use подруга for female friend, and the past tense verbs would become feminine:

• Подруга перевернула картину и разгадала головоломку.

Notice the changes:

• друг → подруга
• перевернул → перевернула
• разгадал → разгадала

The objects stay the same:

• картину
• головоломку

because they are still feminine accusative singular.

The main stresses are:

• друг
• переверну́л
• карти́ну
• и
• разгада́л
• головоло́мку

Stress is important in Russian because it is not always predictable. If you are reading aloud, a natural pronunciation would be:

Друг переверну́л карти́ну и разгада́л головоло́мку.