Онук росте дуже швидко, і дідусь завжди каже про це з усмішкою.

Онук is the nominative singular form, which is the basic dictionary form for grandson. It is the subject of росте, so nominative is required.

A quick contrast:

• онук = grandson
• онука = granddaughter

So in this sentence, онук means the grandson.

Росте is the 3rd person singular present tense of the verb рости, meaning to grow.

So:

• я росту = I grow / am growing
• ти ростеш = you grow
• він / вона / воно росте = he / she / it grows

Since онук is a singular subject, росте is the correct form.

Yes. Both words function adverbially here:

• дуже = very
• швидко = quickly / fast

Together, дуже швидко means very quickly and modifies the verb росте.

This is like English:

• He grows quickly
• He grows very quickly

In Ukrainian, a comma is often used before і when it joins two separate clauses, especially when each clause has its own verb.

Here the two clauses are:

• Онук росте дуже швидко
• дідусь завжди каже про це з усмішкою

Each part has its own subject and verb:

• онук росте
• дідусь каже

So the comma is natural and standard.

Yes. Дідусь is a warmer, more affectionate form.

• дід = grandfather / old man, more plain or rough depending on context
• дідусь = grandpa / granddad, affectionate and common in family speech

So дідусь here sounds natural and warm, like grandpa in English.

Yes, казати usually means to say or to tell, but in real usage it can also work in contexts where English would use talk about or mention.

So каже про це literally looks like says about this, but natural English would usually be:

• talks about it
• mentions it
• sometimes says it with a smile, depending on context

The Ukrainian structure казати про + accusative is normal.

After the preposition про meaning about, Ukrainian normally uses the accusative case.

So:

• про це = about this / about it

A useful thing to know is that це looks the same in both nominative and accusative, so the form does not change here. Even though the case is accusative, it still appears as це.

Yes, говорить про це would also be possible.

Very roughly:

• казати often focuses more on saying something
• говорити often focuses more on speaking / talking

In this sentence:

• каже про це sounds fine and natural
• говорить про це also sounds natural

The difference is subtle here, and both could work depending on style and speaker preference.

Усмішкою is in the instrumental case.

After з meaning with, Ukrainian often uses the instrumental case when expressing accompaniment or manner.

So:

• з усмішкою = with a smile

Base form:

• усмішка = smile

Instrumental singular:

• усмішкою

This is a very common pattern:

• з другом = with a friend
• з радістю = with joy
• з усмішкою = with a smile

They are very close in meaning, and both can mean smile.

In many contexts, усмішка and посмішка are near-synonyms. Depending on speaker, region, and style, one may be preferred over the other. In a sentence like this, either could sound natural:

• з усмішкою
• з посмішкою

Learners should just recognize both as common words for smile.

Завжди means always, and its placement before the verb is very natural in Ukrainian.

So:

• дідусь завжди каже про це = grandpa always talks about it

Ukrainian word order is more flexible than English, but this placement is neutral and common.

You may also see different orders for emphasis, for example:

• Дідусь каже про це завжди
• Завжди дідусь каже про це

But the original sentence sounds the most neutral.

Ukrainian word order is fairly flexible because grammatical roles are often shown by case endings rather than position alone.

The original order is neutral:

• Онук росте дуже швидко, і дідусь завжди каже про це з усмішкою.

Other orders are possible for emphasis, for example:

• Дуже швидко росте онук...
• І дідусь про це завжди каже з усмішкою.

But changing the order can slightly change what is emphasized. For learners, the original version is a good standard pattern.

It can mean either, depending on context.

Ukrainian present tense often covers both:

• grows
• is growing

So Онук росте дуже швидко could be understood as:

• The grandson grows very quickly
• The grandson is growing very quickly

In natural English, is growing very quickly is often the smoother translation in this context.

The main stresses are:

• ону́к
• росте́
• ду́же
• швидко́
• діду́сь
• завжди́
• про це́
• усмі́шкою

Stress is important in Ukrainian, because it is not always predictable. If you are learning the sentence aloud, it is worth memorizing the stressed syllables along with the words.

No. Ukrainian also has та, which can often mean and.

So in many contexts, you could also see:

• Онук росте дуже швидко, та дідусь завжди каже про це з усмішкою.

But і is the most basic and common word for and, and it fits perfectly here.

It suggests a habitual or repeated situation.

Clues:

• завжди = always
• present tense verbs = general ongoing situation

So the idea is not just one single moment. It means something like:

• the grandson is growing quickly
• and grandpa regularly says something about that with a smile

That is why the sentence feels descriptive and warm, rather than like a one-time event.