У місяці тридцять днів.

Because after the preposition у meaning in, Ukrainian uses the locative case here.

• Dictionary form: місяць = month
• Locative singular: у місяці = in a month / in the month

So:

• місяць = nominative singular
• у місяці = locative singular

This is a very common pattern:

• у році = in a year
• у місті = in a city
• у слові = in a word

So місяці is not plural here; it is singular locative.

Yes. У and в are two forms of the same preposition. In this sentence they mean in.

So these are basically the same in meaning:

• У місяці тридцять днів.
• В місяці тридцять днів.

The choice between у and в is usually about sound and ease of pronunciation, not meaning. Ukrainian often switches between them to avoid awkward sound combinations.

Here у місяці sounds very smooth, so it is a natural choice.

Because in Ukrainian, the present-tense verb to be is often left out.

So the sentence literally looks like:

• In a month thirty days.

But the natural English meaning is:

• There are thirty days in a month.

You can add є if you want to make the existence idea more explicit:

• У місяці є тридцять днів.

Both are understandable, but leaving out є is very normal in Ukrainian.

Because after numbers 5 and higher, Ukrainian usually puts the counted noun in the genitive plural.

So:

• тридцять днів = thirty days
• п’ять книг = five books
• сім років = seven years

Here:

• день = day
• днів = genitive plural of день

So тридцять днів is the normal grammatical pattern.

By contrast, дні is the nominative plural form, and it is not the form you normally use after тридцять.

Тридцять is the numeral thirty.

In this sentence it is in its basic form and directly counts the noun:

• тридцять днів = thirty days

For a beginner, the important thing to remember is this pattern:

• 5+ numeral + genitive plural noun

So you learn the whole chunk:

• тридцять днів
• сорок хвилин
• п’ятдесят сторінок

You do not need to think of тридцять as changing here; the key change you notice is in the noun днів.

Ukrainian has no articles like a, an, or the.

So місяць or у місяці can mean things like:

• a month
• the month
• sometimes just month

The exact English translation depends on context.

That is why У місяці тридцять днів can be translated naturally as:

• There are thirty days in a month

even though there is no separate word for a.

No. Ukrainian word order is fairly flexible.

У місяці тридцять днів is a normal, neutral way to say it. But other versions are also possible, depending on emphasis:

• У місяці є тридцять днів.
• Тридцять днів у місяці.
• Місяць має тридцять днів.

They are not all identical in style or emphasis, but they are all understandable.

A rough guide:

• У місяці тридцять днів = neutral statement
• У місяці є тридцять днів = a bit more explicit
• Тридцять днів у місяці = emphasizes thirty days
• Місяць має тридцять днів = uses has

Yes, absolutely.

That sentence means:

• A month has thirty days

It is grammatical and natural. The difference is mostly in how the idea is expressed:

• У місяці тридцять днів = There are thirty days in a month
• Місяць має тридцять днів = A month has thirty days

So one uses an in structure, and the other uses has.

Both are useful patterns in Ukrainian.

No. Even though місяці can also be a plural form in other contexts, here it is singular locative because it follows у.

So in this sentence:

• у місяці = in a month

It does not mean in months here.

This is something English speakers often notice because the ending -і may look plural at first, but Ukrainian case endings can make a singular noun look different from its dictionary form.

Grammatically, yes. It is a normal sentence pattern.

In real life, the statement is a simplified generalization, so you might also hear more precise versions such as:

• У деяких місяцях тридцять днів. = Some months have thirty days.
• У місяці буває тридцять або тридцять один день. = A month can have thirty or thirty-one days.

So the sentence is useful for learning grammar, even if the fact itself is simplified.

Yes. Місяць can mean both month and moon.

That is normal in Ukrainian, and context tells you which meaning is intended.

In this sentence:

• У місяці тридцять днів

the meaning can only be month, because thirty days clearly refers to time.

So although the word has two meanings, there is no real ambiguity here.