Muszę obudzić dzieci o siódmej rano.

Because muszę already tells you the subject is I. It is the 1st person singular form of musieć.

Polish often drops subject pronouns when they are clear from the verb ending. So:

• Muszę obudzić dzieci = I have to wake the children
• Ja muszę obudzić dzieci = also correct, but ja adds emphasis, like I have to do it.

The dictionary form is musieć.

Muszę means I must or I have to. In everyday English, have to is often the most natural translation, but must can also fit depending on context.

Some common forms are:

• muszę = I must / I have to
• musisz = you must / you have to
• musi = he/she/it must

This is about aspect, which is very important in Polish verbs.

• budzić = imperfective: to be waking, to wake repeatedly, to wake in general
• obudzić = perfective: to wake someone up successfully, to bring the action to completion

In this sentence, obudzić is used because the idea is a single completed action: the children need to end up awake.

So:

• Muszę obudzić dzieci = I have to wake the children up
• Muszę budzić dzieci would suggest something more like a repeated/habitual action, for example every school day

It is built into the Polish verb.

English often uses a verb + particle combination, like wake up. Polish does not need a separate word here. The verb obudzić already carries the idea of waking someone up.

So obudzić is not just wake in a vague sense; it means to wake someone so that they are awake.

Dzieci is the plural form of dziecko (child), and here it is the direct object of obudzić, so it is in the accusative.

The important point is that for this word, the nominative plural and accusative plural are both dzieci.

So:

• dzieci śpią = the children are sleeping
• obudzić dzieci = to wake the children

The form looks the same, but the function is different.

Because Polish tells clock time with o + locative.

So when you say at seven, Polish uses a form like:

• o pierwszej = at one
• o drugiej = at two
• o siódmej = at seven

You do not say o siedem for clock time.

Because it comes from the ordinal number siódmy (seventh), not directly from siedem (seven).

Historically and grammatically, the phrase is understood as something like o siódmej godzinie = at the seventh hour. The word godzinie is usually omitted, but it still affects the form:

• godzina is feminine
• after o, you need the locative
• so the form becomes siódmej

That is why you get o siódmej, not o siódmy or o siedem.

Rano is not always necessary, but it is often useful.

• o siódmej = at seven
• o siódmej rano = at seven in the morning

Without rano, the sentence may still be clear from context. But rano removes any doubt and makes it explicit that this is 7 a.m., not 7 p.m.

So it is very natural here.

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

The sentence:

• Muszę obudzić dzieci o siódmej rano

is a neutral, natural version.

But other orders are also possible, with different emphasis:

• O siódmej rano muszę obudzić dzieci.
  Emphasis on the time.

• Dzieci muszę obudzić o siódmej rano.
  Emphasis on the children.

The basic meaning stays the same, but the focus shifts.

This is a very common question.

• obudzić kogoś = to wake someone
• obudzić się = to wake up oneself, to wake up

So:

• Muszę obudzić dzieci = I have to wake the children
• Muszę się obudzić o siódmej rano = I have to wake up at seven in the morning

The się changes the meaning from waking someone else to waking oneself.

Yes, but the meaning changes a little.

• Muszę obudzić dzieci o siódmej rano
  Focuses on one completed action: the children need to be awake.

• Muszę budzić dzieci o siódmej rano
  Sounds more like a repeated duty or routine, for example every weekday.

So if you mean a one-time or complete action, obudzić is the better choice. If you mean a regular habit, budzić may be more appropriate.

Dzieci is pronounced roughly like דזשE-chi to an English speaker, but that is only approximate.

More carefully:

• dz here is part of a soft sound written dź / dzi
• ci at the end sounds like a soft chi
• the stress is on the first syllable: DZIE-ci

A rough learner-friendly approximation is:

• JYEH-chee

But listening to native audio is the best way to get the sound right.