Когда я открываю дверь, собака начинает лаять, а котёнок тихо мяукает.

In this sentence, когда means when / whenever. Russian often uses the present tense to describe something that happens regularly or whenever a situation occurs.

So Когда я открываю дверь... means something like When / Whenever I open the door...

This is a general, repeated situation, not one single event in the future or past.

Открываю is the imperfective present form of открывать. It is used because the sentence describes a repeated or habitual action:

• Когда я открываю дверь... = Whenever I open the door...

If you said Когда я открою дверь..., that would usually refer to one completed future action:

• Когда я открою дверь, ... = When I open / when I have opened the door... in a specific future situation

So открываю fits the idea of a regular pattern.

It actually is in the accusative case, because it is the direct object of открываю. But дверь is a feminine noun whose nominative and accusative singular forms look the same:

• nominative: дверь
• accusative: дверь

So the form does not visibly change, even though the case has changed grammatically.

Both are possible, but they mean slightly different things.

• собака лает = the dog is barking / barks
• собака начинает лаять = the dog starts barking / begins to bark

The sentence focuses on the moment the dog reacts when the door opens. So начинает лаять highlights the beginning of the barking.

After verbs like начинать (to begin / start), Russian normally uses the infinitive of the action that begins.

So:

• начинает лаять = starts to bark
• literally: begins bark-to

This is very similar to English start + infinitive.

The verb мяукать means to meow. Its 3rd person singular form is мяукает:

• котёнок мяукает = the kitten meows / is meowing

Animal-sound verbs in Russian often have special forms that just need to be learned as vocabulary:

• лаять = to bark
• мяукать = to meow
• мычать = to moo
• каркать = to caw

So мяукает is simply the correct present-tense form of мяукать.

Тихо is an adverb, meaning quietly. It describes how the kitten meows.

• котёнок тихо мяукает = the kitten meows quietly

Тихий is an adjective, meaning quiet in the sense of describing a noun:

• тихий котёнок = a quiet kitten

So here you need the adverb тихо, because it modifies the verb мяукает.

Both а and и can connect parts of a sentence, but they are not exactly the same.

• и = and, simple addition
• а = and / while / whereas / but, often showing contrast or a separate parallel action

In this sentence:

• собака начинает лаять, а котёнок тихо мяукает

the idea is that the two animals react differently. The dog starts barking, while the kitten meows quietly. So а works well because it sets the two reactions side by side with a slight contrast.

There are commas for two reasons:

1. Когда я открываю дверь, ...
   A subordinate clause introduced by когда is separated by a comma from the main clause.

2. ..., собака начинает лаять, а котёнок тихо мяукает.
   The two main clauses are joined by а, so Russian puts a comma before а.

So the punctuation is completely normal for Russian sentence structure.

Котёнок is grammatically masculine. Many Russian nouns ending in a consonant are masculine, and котёнок follows that pattern.

Its verb agrees accordingly:

• котёнок мяукает

Even if the actual kitten could be female in real life, the noun котёнок itself is grammatically masculine unless you use a different word.

Yes. Russian word order is more flexible than English, because case endings carry much of the grammatical information. But changing the order changes the emphasis.

The neutral order here is:

• Когда я открываю дверь, собака начинает лаять, а котёнок тихо мяукает.

You could also say:

• Когда я открываю дверь, котёнок тихо мяукает, а собака начинает лаять.

That would still be grammatical, but the focus would shift slightly.

So the original word order is natural and neutral for presenting the situation in a straightforward way.

Most naturally, it describes something that happens regularly or whenever the speaker opens the door.

That is because of the present-tense imperfective verbs:

• открываю
• начинает
• мяукает

Together with когда, this gives the sense of a repeated pattern:

• Whenever I open the door, the dog starts barking and the kitten meows quietly.

If Russian wanted to emphasize one specific future moment, it would normally use different verb forms.