Вони уже відповіли на моє повідомлення.

Вони means they. It is the third-person plural pronoun.

In this sentence, it most likely refers to people: they have already replied...
Like English they, it can also refer to things in some contexts, but here a human meaning is the natural one.

It is capitalized simply because it starts the sentence.

Ukrainian does not have a separate tense that works like the English present perfect (have replied, has done, etc.).

Instead, Ukrainian usually uses the past tense, and the exact English translation depends on context:

• Вони відповіли = They replied / They have replied
• Вони уже відповіли = They have already replied

So відповіли is completely normal here, even though English uses have replied.

Відповіли is the past tense plural form of відповісти.

Here is what it shows:

• past tense: the action happened before now
• plural: it matches Вони = they
• perfective aspect: the reply is seen as a completed action

So відповіли means something like replied / have replied, with a sense of completion.

Compare:

• відповіли = they replied, they have replied
• відповідали = they were replying / they used to reply / they replied (with focus on process or repeated action, depending on context)

Уже and вже both mean already. In most cases, they are interchangeable.

So both of these are correct:

• Вони уже відповіли на моє повідомлення.
• Вони вже відповіли на моє повідомлення.

The choice is often based on:

• rhythm
• pronunciation flow
• personal or stylistic preference

Many learners will hear вже very often in everyday speech, but уже is also standard and natural.

Yes, на often means on, but prepositions do not match English word-for-word.

With the verb відповісти / відповідати, Ukrainian normally uses:

• відповісти на запитання = answer a question
• відповісти на лист = reply to a letter
• відповісти на повідомлення = reply to a message

So in this sentence, на is just part of the normal Ukrainian pattern for reply to something.

Because відповісти на щось requires на + accusative.

So:

• на моє повідомлення = correct

But моєму повідомленню is dative, and that is not the form used for the thing being replied to.

A useful contrast:

• відповісти комусь = reply to someone
• відповісти на щось = reply to something

Examples:

• Вони відповіли мені. = They replied to me.
• Вони відповіли на моє повідомлення. = They replied to my message.
• Вони відповіли мені на моє повідомлення. = They replied to me in response to my message.

Because моє has to agree with повідомлення.

Повідомлення is a neuter singular noun, so the possessive my must also be in the neuter singular form:

• мій лист = my letter/message (masculine)
• моя відповідь = my answer (feminine)
• моє повідомлення = my message (neuter)

This is a very common agreement pattern in Ukrainian.

It is in the accusative singular.

However, for an inanimate neuter noun like повідомлення, the nominative and accusative forms are the same. So even though it is accusative here, it looks unchanged.

That is why you see:

• nominative: це повідомлення = this message
• accusative: на повідомлення = to the message / in reply to the message

Same form, different grammatical role.

Yes. Ukrainian word order is more flexible than English word order.

The sentence as given is a neutral, natural way to say it:

• Вони уже відповіли на моє повідомлення.

But other orders are possible if you want different emphasis:

• На моє повідомлення вони уже відповіли.
  Emphasis on my message
• Уже вони відповіли на моє повідомлення.
  More marked, with emphasis on already
• Вони на моє повідомлення уже відповіли.
  Also possible, but less neutral

So the words can move around more than in English, but the neutral learner-friendly version is the original one.

Sometimes yes, if the context already makes the subject clear.

For example, in conversation:

• Уже відповіли на моє повідомлення.

This can work if everyone already knows who they are.

But there is an important point: in the Ukrainian past tense, the verb does not clearly show person the way English does.
Відповіли could mean, depending on context:

• we replied
• you replied (plural or formal)
• they replied

So keeping Вони often makes the sentence clearer.

Повідомлення usually means message, but depending on context it can also mean:

• a text/message in chat
• a notification
• a report or announcement

In this sentence, the most natural meaning is message, especially something written or electronic.

So learners should remember that повідомлення is a broad word, and context tells you which English translation fits best.