Chomik śpi w dzień, a biega w nocy, więc dzieci karmią go dopiero wieczorem.

In this sentence, a links two contrasting facts:

• śpi w dzień = sleeps during the day
• biega w nocy = runs at night

So a often means something like and, but with a contrast in the background.

Compare:

• i = plain and
• a = and/but with contrast
• ale = stronger but

Here, a is perfect because the two actions are opposed in time: day vs night.

Polish often leaves out the subject when it is already clear from context.

So:

• Chomik śpi w dzień, a biega w nocy
  literally: The hamster sleeps during the day, and runs at night

The second chomik is omitted because it would sound repetitive. Polish does this very naturally.

You can say:

• Chomik śpi w dzień, a chomik biega w nocy

but it sounds unnecessary.

W dzień is a very common expression meaning during the day / in the daytime.

Even though dzień is often taught as day, the phrase w dzień is idiomatic. Here w takes the accusative, not the locative.

So:

• w dzień = during the day
• w nocy = at night

By contrast, w dniu usually means on the day of or sounds more formal/specific, for example:

• w dniu ślubu = on the wedding day

So in everyday speech about a general habit, w dzień is the natural choice.

W nocy is the normal Polish expression for at night.

Here:

• noc = night
• after w, it becomes nocy

This is the locative case.

So the phrase is just something you should learn as a set expression:

• w dzień = during the day
• w nocy = at night

These two expressions do not behave in exactly the same grammatical way, so it is best not to force them into one pattern.

Wieczorem is the standard adverb-like form meaning in the evening.

It comes from wieczór and is in the instrumental case, but in practice learners usually memorize it as a fixed time expression:

• rano = in the morning
• wieczorem = in the evening
• nocą / w nocy = at night

So:

• dopiero wieczorem = only in the evening / not until the evening

Using w wieczór would usually sound wrong in this meaning.

Więc means so, therefore, or thus.

It introduces the result or consequence of what came before:

• The hamster sleeps during the day and is active at night,
• so the children feed it only in the evening.

So więc connects cause and result.

A close equivalent is więc = so.

Because dzieci is plural and karmią is the 3rd person plural present tense form of karmić.

• dziecko = child
• dzieci = children
• karmią = they feed

So:

• dzieci karmią = the children feed

Even though dzieci does not end in a typical masculine plural ending, it still takes a plural verb.

Go means him/it here and refers to chomik.

The verb karmić takes a direct object in the accusative case, so the pronoun must also be accusative.

• chomik = hamster
• karmią chomika = they feed the hamster
• karmią go = they feed him/it

Because chomik is a masculine animate noun, the accusative pronoun is go.

Polish grammatical gender and English natural gender do not match perfectly.

Chomik is grammatically masculine animate, so Polish uses forms like those used for he/him-type nouns:

• on = he
• go = him

But in English, animals are often called it, especially in general statements.

So go may correspond to:

• him if you think of the hamster as a male animal
• it in neutral English

Both are possible depending on translation style.

Because the sentence describes a habit or routine, not something happening right now only once.

In Polish, the present tense is often used for habitual actions:

• Chomik śpi w dzień = The hamster sleeps during the day
• dzieci karmią go dopiero wieczorem = the children feed it only in the evening

This is like English simple present for routines and general truths.

Dopiero is an important word. Here it means:

• only
• not until

So:

• dzieci karmią go dopiero wieczorem

means the children feed it only in the evening or not until evening.

It suggests that the feeding happens later than you might expect.

Examples:

• Przyszedł dopiero o dziesiątej. = He only came at ten / He didn’t come until ten.
• Zrozumiałem dopiero teraz. = I only understood now / I didn’t understand until now.

Yes, karmić is imperfective.

That matters because the sentence describes a repeated, ongoing habit:

• dzieci karmią go dopiero wieczorem = they feed him/it in the evening as a routine

If you used a perfective verb, it would not fit normal present-tense habitual meaning in the same way.

The same is true of:

• śpi from spać (imperfective)
• biega from biegać (imperfective/frequentative-style habitual verb)

All three verbs suit a general pattern or repeated behavior.

Biega means runs or runs around. It suggests active movement, often repeated or habitual.

For a hamster, biega w nocy sounds natural because hamsters are active and move around at night.

Compare:

• idzie = goes / walks
• biega = runs / runs around
• leci = flies, or sometimes colloquially goes fast

So biega gives the right image of energetic nighttime activity.

Chomik is in the nominative case because it is the subject of the sentence.

• Chomik śpi = The hamster sleeps

When the same animal becomes the object later, the form changes:

• dzieci karmią chomika = the children feed the hamster

So:

• chomik = nominative
• chomika / go = accusative

This is a useful example of how Polish marks subject vs object.

Yes, absolutely.

Both are correct:

• dzieci karmią chomika dopiero wieczorem
• dzieci karmią go dopiero wieczorem

Using go avoids repeating chomika, so it sounds smoother and more natural after chomik has already been mentioned.

Not completely. Polish word order is flexible, though some versions sound more natural than others.

Possible variants include:

• Dzieci karmią go dopiero wieczorem.
• Dopiero wieczorem dzieci karmią go.
• Go dzieci karmią dopiero wieczorem. — possible, but more marked

The original order is neutral and natural. It puts the basic information first and the time phrase at the end.

There actually is a comma before both of them in the sentence:

• Chomik śpi w dzień, a biega w nocy, więc dzieci karmią go dopiero wieczorem.

In Polish, commas are normally used before conjunctions like:

• a
• ale
• więc
• że
• bo

So the punctuation here is standard and correct.

It sounds like a general rule or habit.

That comes from:

• the present tense verbs: śpi, biega, karmią
• the general time expressions: w dzień, w nocy, wieczorem

So the meaning is not today the children are feeding the hamster in the evening, but rather that is when they usually feed it.