Моя дочь любит смузи с манго, а сын предпочитает обычный сок.

Because дочь is a feminine noun, and Russian possessive pronouns must agree with the noun they describe.

• мой = masculine singular
• моя = feminine singular
• моё = neuter singular
• мои = plural

So with дочь, you need моя: моя дочь.

Not all feminine nouns end in -а or -я. Some feminine nouns end in a soft sign ь, and дочь is one of them.

This is just a noun pattern you have to learn. Other common feminine nouns with ь include:

• ночь = night
• мышь = mouse
• дверь = door

So even though дочь ends in ь, it is still feminine.

Russian often leaves out repeated words when the meaning is already clear.

So:

• Моя дочь любит смузи с манго, а сын предпочитает обычный сок.

naturally means:

• My daughter likes mango smoothies, and my son prefers regular juice.

You could also say а мой сын предпочитает..., and that would be correct too. Adding мой makes it a little more explicit or slightly more contrastive.

А is very common when Russian compares or contrasts two parallel ideas.

Here the two parts are:

• моя дочь любит смузи с манго
• сын предпочитает обычный сок

This is not a strong contradiction, so но would sound too strong.
And и would sound more like simple addition, without highlighting the contrast.

So а works well for:

• X likes this, whereas Y prefers that

That is exactly the feeling in this sentence.

It is the direct object of любит, so it is in the accusative case.

The verb любить normally takes the accusative:

• любить чай
• любить музыку
• любить смузи

The reason смузи looks unchanged is that it is an indeclinable loanword, so its form stays the same.

Because смузи is an indeclinable borrowed word in Russian. That means it keeps the same form in different cases.

So even though it is the object here, it still appears as смузи.

This is common with some modern foreign words in Russian, especially food and drink names.

In this sentence, с means with, in the sense of containing or flavored with:

• смузи с манго = smoothie with mango / mango smoothie

Normally, с requires the instrumental case. But манго is also an indeclinable word, so its form does not change. That is why you see с манго, not something like с мангом.

So the grammar is still doing what you expect; the word just happens to be indeclinable.

It is also in the accusative case, because предпочитать takes a direct object.

• предпочитать кофе
• предпочитать чай
• предпочитать обычный сок

The reason it looks like the dictionary form is that сок is a masculine inanimate noun. For masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: обычный сок
• accusative: обычный сок

No visible change happens here.

Обычный means ordinary, regular, or plain, and it fits this context very naturally.

Here it contrasts with something more special or trendy, like смузи с манго. So обычный сок gives the idea of regular juice.

You could sometimes use обыкновенный, but обычный is the more natural everyday choice here.

Also, обычный agrees with сок:

• masculine
• singular
• accusative inanimate (same form as nominative)

So обычный сок is grammatically and stylistically natural.

Both are present tense imperfective forms.

That makes sense because the sentence describes a general preference, not a one-time action:

• любит = likes / loves
• предпочитает = prefers

Russian uses the imperfective for habits, general truths, and ongoing states like preferences.

So this sentence is not about what they are choosing right now; it is about what they generally like.

Yes. Russian word order is fairly flexible, but the version you have is the most neutral and natural.

• Моя дочь любит смузи с манго, а сын предпочитает обычный сок.

This order simply presents:

1. the subject
2. the verb
3. the object

If you changed the order, you would usually add emphasis or contrast. For example:

• Смузи с манго моя дочь любит, а сын предпочитает обычный сок.

That sounds more marked, with extra emphasis on смузи с манго.

So the original sentence is the best neutral pattern for a learner to copy.

The main stress pattern is:

моя́ дочь лю́бит сму́зи с ма́нго, а сын предпочита́ет обы́чный сок

A few useful notes:

• моя́ — stress on the last syllable
• лю́бит — stress on the first syllable
• сму́зи — stress on the first syllable
• ма́нго — stress on the first syllable
• предпочита́ет — stress on -та-
• обы́чный — stress on бы

Learning the stress together with the word is very important in Russian.