Белый голубь сидел на крыше, а потом полетел к реке.

Because белый has to agree with голубь in gender, number, and case.

• голубь is masculine
• it is singular
• it is the subject, so it is in the nominative case

So the adjective also takes the masculine singular nominative form:

• белый голубь = white pigeon/dove

If the noun changed, the adjective would change too:

• белая река = white river
• белое облако = white cloud
• белые дома = white houses

In Russian, a noun ending in ь can be either masculine or feminine. The soft sign ь by itself does not tell you the gender.

Голубь is a masculine noun, so other words in the sentence behave accordingly:

• белый is masculine
• сидел is masculine past tense
• полетел is masculine past tense

This is something learners often just have to memorize noun by noun.

The ь does not have its own sound. It mainly shows that the preceding consonant is soft.

So in голубь, the final б is pronounced softly.

It can also help distinguish grammatical forms and noun types, but here its main effect for pronunciation is softness.

Russian past tense agrees with the subject in gender and number.

Since голубь is masculine singular, the past tense form is:

• сидел = he/it sat

Compare:

• голубка сидела = the female dove sat
• окно сидело would be impossible semantically, but grammatically сидело is neuter
• голуби сидели = the pigeons sat

So сидел tells you the subject is masculine singular.

A very common pattern in Russian is:

• verb stem + -л = past tense base

Then the ending reflects gender/number:

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

So:

• сидел = masculine past of сидеть
• полетел = masculine past of полететь

In this sentence, both verbs agree with the same masculine singular subject, голубь.

Because the preposition на here means on in the sense of location, and with that meaning it takes the prepositional case.

• dictionary form: крыша
• prepositional singular: на крыше

So:

• на крыше = on the roof

This is different from motion onto a surface:

• на крышу = onto the roof

So Russian distinguishes:

• where? → usually prepositional after на
• to where? onto what? → often accusative after на

Because the preposition к always takes the dative case.

• dictionary form: река
• dative singular: реке

So:

• к реке = to/toward the river

This is a very important pattern to memorize:

• к дому = toward the house
• к маме = to mom
• к морю = toward the sea

They express different kinds of movement.

• к реке = to / toward the river
  • the bird flies in the direction of the river
  • it does not necessarily enter it
• в реку = into the river
  • this would mean movement into the water

So к is about approaching a destination, while в is about going inside something.

This is a very common aspect question.

• летел is imperfective
• полетел is perfective

In this sentence, полетел suggests a single completed action or the idea that the bird set off / flew off toward the river.

Very roughly:

• летел = was flying / flew, focusing on the process
• полетел = started flying and went off, focusing on the event as a whole

The prefix по- often gives that sense of setting out or viewing the action as a whole.

So the sentence sounds natural as a sequence of completed events:

• it sat on the roof
• then it flew off toward the river

Потом means then / afterwards.

The conjunction а links the two clauses. Here it gives a natural transition, something like:

• and then
• and after that
• sometimes with a light sense of contrast or change of scene

So:

• ..., а потом полетел к реке = ..., and then it flew to/toward the river

You could sometimes omit а, but а потом sounds very natural in narration because it clearly moves the story to the next event.

Because а is a coordinating conjunction joining two clauses, each with its own verb:

• Белый голубь сидел на крыше
• а потом полетел к реке

Russian normally puts a comma before а in this kind of sentence.

So the comma is not optional here in standard writing.

It can mean either, depending on context.

Russian does not always separate pigeon and dove the way English often does. Голубь is the general word.

So in some contexts:

• голубь = pigeon
• голубь = dove

If the meaning has already been given to the learner, then the main thing to notice here is the grammar, not a strict English species distinction.

Yes. Russian word order is fairly flexible because case endings carry a lot of the grammatical information.

The neutral order here is:

• Белый голубь сидел на крыше, а потом полетел к реке.

But you could also say:

• На крыше сидел белый голубь, а потом полетел к реке.

That version emphasizes where the bird was.

Even when word order changes, the forms still show the relationships:

• голубь is still the subject
• на крыше is still location
• к реке is still direction toward the river

A natural stress pattern is:

• Бе́лый го́лубь сиде́л на кры́ше, а пото́м полете́л к реке́.

A few pronunciation notes:

• белый → stress on the first syllable
• голубь → stress on го́
• сидел → stress on the last syllable
• крыше → stress on кры́
• потом → stress on the second syllable
• полетел → stress on the last syllable
• реке → stress on the last syllable

Listening and repeating whole chunks like на кры́ше and к реке́ is especially helpful.

Russian has no articles like English the or a/an.

So:

• Белый голубь can mean a white pigeon/dove or the white pigeon/dove
• the exact meaning depends on context

Russian usually expresses definiteness through context, word order, and shared knowledge rather than articles.