На диване дремала кошка, а я тихо писала сообщение.

Because both subjects are feminine.

• кошка = cat, and it is a feminine noun, so the past tense verb is feminine: дремала
• я can mean either I (male) or I (female), but here the speaker is female, so we get писала

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

• masculine: писал
• feminine: писала
• neuter: писало
• plural: писали

So a male speaker would say я тихо писал сообщение.

Because кошка is the subject of the verb, while диване is part of a prepositional phrase.

• кошка answers who was dozing? → subject → nominative
• на диване means on the sofa/couch → location → preposition на
  • prepositional case

So:

• диван = nominative dictionary form
• на диване = prepositional case after на when expressing location

This is a very common pattern:

• на столе = on the table
• в комнате = in the room
• на диване = on the sofa

Russian word order is flexible, and the beginning of the sentence often shows what the speaker wants to set as the scene or emphasize first.

Starting with На диване puts the location first, almost like:

• On the sofa, a cat was dozing...

This sounds natural in Russian. It creates the setting before introducing the subject.

Compare:

• Кошка дремала на диване = neutral, straightforward
• На диване дремала кошка = more scene-setting, often a bit more descriptive or literary

Both are grammatical.

Again, this is because Russian word order is flexible.

In English, the subject usually comes before the verb: The cat was dozing.
In Russian, the subject can come after the verb for stylistic or informational reasons.

Here, дремала кошка sounds natural because the sentence first sets the location, then gives the action, then identifies who was doing it.

This often happens when the subject is being introduced as part of the scene:

• На улице стояла машина. = There was a car standing outside / A car was parked outside.
• В комнате сидел человек. = A person was sitting in the room.

So дремала кошка is not unusual at all.

А often connects two clauses with a slight contrast, comparison, or shift of focus, while и is a more neutral and.

Here the sentence contrasts two simultaneous actions:

• the cat was dozing
• I was quietly writing a message

It is not a strong opposition like but, but there is a mild contrast: the cat was doing one thing, while I was doing another.

So а works very naturally here.

Compare:

• ..., а я... = ..., while I...
• ..., и я... = ..., and I...

Using и would also be possible in some contexts, but а better captures the sense of whereas / while.

Because Russian uses the past tense for actions that were happening in the past, and the imperfective aspect shows that they were ongoing, repeated, or in progress.

So:

• дремала = was dozing / dozed lightly
• писала = was writing

The tense is past, but the aspect shows the action as ongoing or unfinished.

This is similar to the English past continuous, although Russian does not form it with a separate was/were + -ing structure.

Because the sentence describes background actions in progress, not completed results.

• дремать is naturally imperfective: it means to doze, a process or state
• писать is imperfective: to write
• the perfective partner of писать is usually написать = to write and finish

Here the speaker is describing what was going on at that moment, so imperfective is exactly what Russian normally uses.

If you used a perfective verb like написала, the meaning would shift to completion:

• я тихо написала сообщение = I quietly wrote/finished a message

That is different from I was quietly writing a message.

The difference is aspect:

• писала сообщение = was writing a message / wrote messages as a process
• написала сообщение = wrote and completed the message

So in this sentence, писала focuses on the activity itself, not the finished result.

This is one of the most important Russian aspect contrasts:

• писать = to write, be writing
• написать = to write and finish

It is in the accusative because it is the direct object of писала.

The speaker was writing what?
→ сообщение

For many neuter inanimate nouns in Russian, the accusative singular is identical to the nominative singular.

So:

• nominative: сообщение
• accusative: сообщение

That is why the form does not change here.

Тихо is an adverb meaning quietly, softly, or sometimes silently/calmly, depending on context.

Here it modifies писала:

• я тихо писала сообщение = I was quietly writing a message

It tells us how the action was done.

It comes from the adjective тихий = quiet.

Compare:

• тихий голос = a quiet voice
• говорить тихо = to speak quietly
• писать тихо = to write quietly

In this sentence, it creates a calm, subdued atmosphere.

Sometimes yes, but here keeping я is very natural and helpful.

Russian often omits subject pronouns when the subject is clear from the verb form or context. However, in the past tense, the verb does not show person clearly the way present tense does. Also, here я helps create the contrast with кошка.

So:

• ..., а я тихо писала сообщение = natural and clear
• ..., а тихо писала сообщение = possible only in the right context, but less clear and less natural here

Including я makes the sentence easier and more balanced.

Yes.

• спать = to sleep
• дремать = to doze, nap lightly, be half asleep

So дремала кошка suggests the cat was not deeply asleep, but rather lightly dozing.

This choice gives the sentence a softer, more vivid feeling.

Yes, that would be grammatical, but it would sound different in style.

Your version:

• Кошка тихо дремала на диване, а я тихо писала сообщение

This is more straightforward and neutral.

The original:

• На диване дремала кошка, а я тихо писала сообщение

This is more descriptive and scene-based. It first sets the location, then presents the cat as part of the scene.

Russian often uses word order not just for grammar, but for tone, rhythm, and emphasis.

Because а joins two clauses, each with its own subject and verb:

1. На диване дремала кошка
2. я тихо писала сообщение

In Russian, clauses joined by coordinating conjunctions like а, но, and often и are usually separated by a comma.

So the comma is required here.