Learn Hardcore Japanese: tyourei no toki, watasi ha minna to issyo ni tatimasu. - At the morning assembly, I stand together with everyone.

In schools: a short gathering in the morning, often with the whole class, grade, or school. There may be announcements, greetings, sometimes a short talk from a teacher or principal.
In companies: a brief stand-up meeting at the start of the workday. It can include greetings, reporting, checking goals, slogans, etc.

So it’s usually translated as “morning assembly” or “morning meeting”, but it implies this formal, routine gathering, not just any random meeting that happens to be in the morning.

Why is の used in 朝礼のとき? Why not 朝礼で or something else?

Noun + の + とき is a standard pattern meaning “when …” / “at the time of …”.

朝礼のとき literally: “the time of the morning assembly” → “when there is morning assembly / during morning assembly”

If you said:

朝礼で – this uses で as a location or situation marker, so it would mean something like “at the morning assembly (place/event)”, focusing on where the action happens.
朝礼に – could mean “at morning assembly / for morning assembly”, but with とき, the natural form is 朝礼のとき, not 朝礼にとき.

So the pattern here is:

Noun + のとき = when (noun happens) / when it is (noun)
- 子どものとき – when I was a child
- 休みのとき – when it’s a day off
- 朝礼のとき – when we have morning assembly

What does とき add here? Could I just say 朝礼、私は皆と一緒に立ちます?

とき means “when” / “at the time” and explicitly marks the time frame of the action.

朝礼、私は皆と一緒に立ちます is not natural; it sounds like a list, missing the “when” connection.
朝礼のとき、… clearly means “When it’s morning assembly, …” / “During morning assembly, …”

Without とき, you would need a different structure, for example:

朝礼では、私は皆と一緒に立ちます。 – “At morning assembly, I stand together with everyone.”

So とき is the word that creates the “when …” clause.

Why is は used after 私? What would change if it were 私が?

は is the topic marker. It marks what the sentence is about:

私は皆と一緒に立ちます。
→ “As for me, I stand together with everyone.”

If you used が:

私が皆と一緒に立ちます。
→ Emphasis: “I am the one who stands together with everyone.” (implying contrast, like not someone else.)

In this neutral sentence:

私は is natural because you are just introducing what you usually do.
私が would sound like you’re contrasting yourself with others or answering “Who is it that stands…?”

What is the difference between 皆 and みんな? Why is 皆 used here?

皆（みな） and みんな are very closely related:

皆（みな） – slightly more formal, often used in writing, announcements, or polite speech.
みんな – more casual, very common in everyday conversation.

Both generally mean “everyone / all (of you/them/us)”.

In this sentence:

皆 fits the polite style of 立ちます and the formal situation of 朝礼.
You might hear みんなと一緒に立ちます in casual speech, but 皆と一緒に立ちます sounds a bit more formal/polite.

Why is the particle と after 皆? Does it mean “and” or “with” here?

Here, と after 皆 means “with”:

A と一緒に = “together with A”

So:

皆と一緒に = “together with everyone”

The particle と can mean “and” when joining nouns (AとB), but with 一緒に, it’s understood as “with”:

友だちと一緒に行きます。 – “I go with my friend.”
家族と一緒に住んでいます。 – “I live with my family.”

So 皆と一緒に立ちます = “I stand with everyone / together with everyone.”

Why is に used after 一緒 in 一緒に? Can I drop に?

一緒に is basically a fixed expression:

一緒に = “together (with someone) / together”

Grammar-wise, に marks the manner or accompaniment here, but you should mainly remember:

Always use 一緒に, not just 一緒 before the verb.

So:

Correct: 皆と一緒に立ちます。
Incorrect/unnatural: 皆と一緒立ちます。

Some set phrases just always take に:

静かに話す – speak quietly
ゆっくり歩く – walk slowly
一緒に行く – go together

In the same way, think of 一緒に as one unit.

Does 立ちます mean “stand up” or “be standing”?

立ちます (polite form of 立つ) generally means “to stand / to stand up”, depending on context.

If you’re sitting and then 立ちます → “stand up”.
If the context is about what people do during an event, it can be closer to “(to) stand” as the action or state there.

In 朝礼のとき、私は皆と一緒に立ちます。:

It suggests a habitual action: “When it’s morning assembly, I (get up and) stand together with everyone.”
In English, you might say “I stand” or “I stand up”; Japanese 立ちます covers both, so the exact translation depends on what sounds natural in English.

Why is the verb 立ちます in ます-form? What nuance does that give?

立ちます is the polite present/future form of 立つ.

Dictionary/plain form: 立つ
Polite form: 立ちます

Using 〜ます style makes the sentence:

Polite and neutral, suitable for talking to teachers, coworkers, strangers, or in writing.

So:

朝礼のとき、私は皆と一緒に立ちます。 – polite, neutral
朝礼のとき、俺はみんなと一緒に立つ。 – casual, male-ish, used with friends, etc.

The present ます-form in Japanese usually covers:

Habitual actions: things you regularly do → that’s the case here.
Future actions: “I will …”

Here it clearly means a habit/routine during morning assembly.

Can I omit 私 in this sentence? Would it still be correct?

Yes, you can omit 私:

朝礼のとき、皆と一緒に立ちます。

Japanese often drops the subject when it’s obvious from context. If you are talking about your own routine, listeners will naturally understand that you are the one standing.

Including 私は:

Adds clarity if the subject might be ambiguous.
Slightly emphasizes “as for me”.

Both versions are grammatically correct. Context decides whether you need 私 or can drop it.

Can I change the word order, like 私は朝礼のとき皆と一緒に立ちます? Is that okay?

Yes, Japanese word order is relatively flexible as long as the particles stay attached to the right words.

These are all natural:

朝礼のとき、私は皆と一緒に立ちます。
私は朝礼のとき、皆と一緒に立ちます。
朝礼のとき、皆と一緒に私は立ちます。 (slight emphasis on 私)

The main rule: the verb comes at the end, and particles make the relationships clear:

朝礼のとき – time
私は – topic (I)
皆と一緒に – “together with everyone”
立ちます – verb

So you can rearrange the chunks, but the meaning stays the same as long as particles are correct.