Дитина боїться грому уночі.

Because Ukrainian often uses case endings where English uses a preposition.

The verb боятися means to be afraid, and it normally takes the genitive case. So instead of saying afraid of thunder with a separate word for of, Ukrainian changes грім to грому.

So:

• грім = thunder
• грому = of thunder / thunder in the genitive after боятися

Грому is the genitive singular form of грім.

This is the form required after боятися. So:

• dictionary form: грім
• after боятися: грому

You may also notice a stem change:

• грім
• грому

That kind of vowel change happens in some Ukrainian nouns when they go into other cases.

Боїться is the 3rd person singular present tense form of боятися.

It matches the subject дитина because дитина is singular.

A few present-tense forms:

• я боюся = I am afraid
• ти боїшся = you are afraid
• він / вона / дитина боїться = he / she / the child is afraid

-ся is a very common Ukrainian verb ending often called a reflexive marker.

In some verbs it has a clearly reflexive meaning, but in боятися it is simply part of the verb’s normal form. So you should learn:

• боятися = to be afraid

not as a literal to fear oneself, but just as the standard verb.

Дитина is grammatically feminine, because it ends in -а.

But it does not only mean a girl. It can mean a child of either sex.

That means Ukrainian grammar treats it as feminine even if the child is a boy. For example, agreement with adjectives or past-tense verbs usually follows the grammatical gender of дитина.

Уночі means at night or during the night.

Grammatically, it functions as an adverbial time expression. For a learner, the easiest thing is to treat уночі as a set word meaning at night.

It is usually written as one word:

• уночі
• also commonly вночі

Both are normal.

Both уночі and вночі are correct and mean the same thing.

Ukrainian often alternates у and в for euphony—in other words, to make the sentence flow more smoothly. The choice can depend on surrounding sounds, rhythm, and speaker preference.

So here, уночі is fine, and вночі would also be fine in many contexts.

Yes. Ukrainian word order is fairly flexible because the case endings show the grammatical roles.

The sentence as given,

• Дитина боїться грому уночі.

is a neutral order.

But you could also say:

• Уночі дитина боїться грому.
  This puts more focus on at night.

• Грому дитина боїться уночі.
  This gives stronger emphasis to thunder.

So the basic meaning stays similar, but the focus changes.

Ukrainian has no articles.

So where English says:

• the child
• a child

Ukrainian simply says:

• дитина

Context tells you whether it means a child, the child, or sometimes just child in a general sense.

It can mean either, depending on context.

Without extra context, this sentence often sounds like a general or habitual statement:

• the child is afraid of thunder at night

But in the right situation, it could also describe what is happening now.

Ukrainian present tense often works like English present tense in this way: context decides whether it is general, habitual, or current.

A couple of things matter here:

• ї is pronounced separately, roughly like yi
• боїться has the sequence бо-ї-
• written -ться is often pronounced close to -цця in natural speech

So боїться is pronounced approximately like:

• bo-yi-tsya

The important part for a learner is: do not merge о and ї into one sound. Keep ї distinct.