Я легла на шезлонг в тени, а подруга осталась на солнце и читала книгу.

Because легла describes a change of position: the speaker lay down or got onto the deckchair.

• лечь → легла = to lie down, to go into a lying position
• лежать → лежала = to be lying, to already be in that position

So:

• Я легла на шезлонг = I lay down on the deckchair
• Я лежала на шезлонге = I was lying on the deckchair

In this sentence, the action is the moment of settling onto the chair, so легла is the right choice.

Because it is past tense feminine singular.

In Russian past tense, the verb agrees with the subject in gender and number:

• masculine: я лёг
• feminine: я легла
• plural: мы легли

So the sentence tells us that the speaker is female.

Because Russian uses different cases depending on whether you mean movement toward a place or location in a place.

With на:

• на + accusative = motion onto something
• на + prepositional = being on something

So:

• легла на шезлонг = lay down onto the deckchair
• лежала на шезлонге = was lying on the deckchair

Here there is movement into a new position, so шезлонг is in the accusative. Since шезлонг is an inanimate masculine noun, its accusative looks the same as the nominative.

These are standard Russian expressions:

• в тени = in the shade
• на солнце = in the sun

English speakers often expect both to work the same way, but Russian uses different prepositions idiomatically here. It is best to learn them as set phrases.

So you say:

• сидеть в тени
• стоять на солнце
• загорать на солнце

Even though на солнце looks literally like on the sun, it simply means in the sun in normal Russian.

Because after в in a location meaning, Russian uses the prepositional case.

The noun тень changes like this:

• nominative: тень
• prepositional: в тени

So в тени means in the shade.

This form тени can look confusing because it is also used in some other cases, but here it is specifically prepositional singular.

Осталась comes from остаться, meaning to remain or to stay behind.

It fits the contrast in the sentence:

• the speaker changed position and moved into the shade
• the friend remained in the sun

So подруга осталась на солнце means something like:

• my friend stayed in the sun
• my friend remained in the sun

It is more precise than simply using была, because осталась emphasizes that she continued to be there instead of moving elsewhere.

Because читала presents the reading as an ongoing activity, not as a completed result.

• читала = was reading / read
• прочитала = finished reading

In this sentence, the reading is background information about what the friend was doing while she stayed in the sun. The point is not that she finished the book, only that she was engaged in reading.

If you said прочитала книгу, it would usually mean she read the whole book from start to finish.

This is a very common Russian pattern.

• легла and осталась are completed, single events
• читала is an ongoing process

So the sentence mixes:

1. actions that move the story forward

   • легла
   • осталась

2. a background activity

   • читала

This is very natural in Russian narration. It is similar to English using a simple past for the main events and a continuous idea for the background:

• I lay down in the shade
• my friend stayed in the sun
• and she was reading a book

Because а often links two ideas that are contrasted or set side by side.

Here the sentence contrasts two people:

• Я in the shade
• подруга in the sun

So а works well because it means something like:

• while
• whereas
• and as for
• but, in a mild contrastive sense

If you used и, it would sound more like simple addition, with less contrast.

Because it is the direct object of читала, so it takes the accusative case.

The noun changes like this:

• nominative: книга
• accusative singular: книгу

So:

• читала книгу = was reading a book

This is a regular pattern for most feminine nouns ending in -а: -а → -у in the accusative singular.

Russian often omits possessive words like мой, моя, его, её when the meaning is already clear from context.

So подруга here can naturally mean my friend if the context already makes that obvious.

Russian does this much more often than English. English usually needs my friend, but Russian is often happy with just подруга.

Because Russian has no articles.

So nouns like шезлонг, подруга, and книгу do not automatically show whether they mean:

• a deckchair / the deckchair
• a friend / the friend
• a book / the book

Russian leaves that to context, word order, and the situation. In this sentence, English would normally add articles in translation, but Russian does not need them.

Yes. Russian word order is flexible, and changing it changes the focus or emphasis, not the basic meaning.

For example, you could also say:

• В тени я легла на шезлонг, а подруга осталась на солнце и читала книгу.

That version puts extra focus on в тени.

The original order is natural and neutral for telling a short story:

• first the subject
• then the action
• then the location
• then a contrasting clause about the friend