Моїй подрузі вісімнадцять років.

Because Ukrainian expresses age with the person in the dative case.

The basic form is:

• моя подруга = my female friend

But in this sentence it becomes:

• моїй подрузі = to my female friend

So both words change:

• моя → моїй
• подруга → подрузі

This is one of the most important differences from English.

Because Ukrainian commonly says age as something like:

• To my friend, eighteen years.

That is the normal Ukrainian pattern for age:

• Мені двадцять років. = I am twenty years old.
• Йому десять років. = He is ten years old.
• Моїй подрузі вісімнадцять років. = My friend is eighteen years old.

So the dative answers the idea to whom is this age?

In the present tense, Ukrainian usually leaves out є (is/are) in sentences like this.

So:

• Моїй подрузі вісімнадцять років.

literally has no spoken is, but it is still a complete sentence.

This is very normal in Ukrainian. English needs is, but Ukrainian usually does not.

A very literal breakdown is:

• Моїй = to my
• подрузі = friend
• вісімнадцять = eighteen
• років = years

So literally it is roughly:

• To my friend, eighteen years.

That sounds unnatural in English, but it reflects the Ukrainian grammar.

Because Ukrainian often changes the final consonant of a noun before certain endings.

Here:

• подруга → подрузі

The г changes to з before -і. This kind of consonant alternation is common in Ukrainian.

You will also see similar changes in other words, for example:

• нога → нозі
• книга → книзі

So подрузі is the correct dative singular form.

Because after 5 and higher, Ukrainian normally uses the genitive plural form of the noun.

With рік (year), the pattern is:

• 1 рік
• 2, 3, 4 роки
• 5+ років

Since 18 belongs to the 5+ group, you get:

• вісімнадцять років

So this is exactly the expected form.

In standard Ukrainian, that is not the normal way to express age.

The natural pattern is:

• Моїй подрузі вісімнадцять років.

or very often simply:

• Моїй подрузі вісімнадцять.

Using мати (to have) for age sounds like a translation from English and is generally not the standard structure learners should use.

It can mean either, depending on context.

Most basically:

• подруга = female friend

But in some contexts it can also mean:

• girlfriend / romantic partner

In this sentence by itself, the safest neutral reading is my female friend unless the surrounding context clearly shows it means girlfriend.

Yes, very often you can.

Both are possible:

• Моїй подрузі вісімнадцять років.
• Моїй подрузі вісімнадцять.

The version with років is a bit more explicit and complete.
The shorter version is very common in everyday speech.

Yes. Ukrainian word order is more flexible than English.

The neutral order here is:

• Моїй подрузі вісімнадцять років.

But you may also hear:

• Моїй подрузі вісімнадцять.
• Вісімнадцять років моїй подрузі.

Changing the order can affect emphasis, but the basic meaning stays the same because the case endings show the grammatical roles.

In Ukrainian, ї represents a sound like yi.

So моїй is pronounced roughly like:

• mo-YIY

Important point: ї is pronounced as its own sound. It does not just soften the previous consonant.

So:

• моїй is not one smooth English-style vowel
• it is more like mo-yi-y

This letter often stands out to English speakers, so it is worth paying attention to.

The usual stress is:

• мої́й
• подру́зі
• вісімна́дцять
• ро́ків

So the sentence sounds roughly like:

• mo-YIY po-DRU-zi vi-sim-NA-dtsyat RO-kiv

Learning stress is useful because Ukrainian stress is not always predictable from spelling.

Here вісімнадцять is a cardinal number: eighteen.

In this sentence it does not change for gender. The age word itself stays вісімнадцять.

The gender and case information is shown elsewhere:

• in моїй подрузі through the dative feminine forms
• in років through the noun form required after the number

So you do not need a special feminine form of eighteen here.