syuuritantousya ha asita kimasu node, tatiaimasu.

修理担当者 literally means the person in charge of repairs.

Breakdown:

• 修理 = repair
• 担当者 = person in charge / person responsible

So in natural English, this could be:

• the repair person
• the technician
• the person handling the repair

It sounds a bit formal or businesslike compared with a very casual word like 修理の人.

は marks 修理担当者 as the topic of the sentence: as for the repair person...

So the first part is setting up what we are talking about:

• 修理担当者は明日来ますので = As for the repair person, they will come tomorrow, so...

Using は often gives a feeling of introducing known or relevant information as the topic.

If you used が instead, it would focus more directly on who is coming:

• 修理担当者が明日来ますので、立ち会います。

That is also possible, but は sounds more like you are talking about the repair person as the topic of the situation.

Words like 明日 often work as time expressions, and many common time expressions can appear without a particle.

So:

• 明日来ます = will come tomorrow

This is very normal Japanese.

You could sometimes see 明日に, but in this kind of sentence it would usually sound unnatural. For basic time words like:

• 今日
• 明日
• 昨日
• 来年

no particle is often used.

来ます is the polite form of 来る.

So:

• 来る = plain form
• 来ます = polite form

The sentence also ends with 立ち会います, which is polite too, so the whole sentence is in a consistent polite style.

This is very common in:

• customer service
• workplace situations
• talking to someone you are not very casual with

ので means because / since / so and connects the reason to the result.

Here:

• 修理担当者は明日来ますので、立ち会います。 = Because the repair person is coming tomorrow, I will be there / I will attend.

The structure is:

• reason + ので + result

So:

• repair person will come tomorrow = reason
• I will be present = result

Compared with から, ので often sounds a bit softer, more explanatory, and slightly more formal.

Both can mean because, but the nuance is a little different.

• ので: softer, more explanatory, often more polite
• から: more direct, sometimes more assertive

Compare:

• 修理担当者は明日来ますので、立ち会います。
• 修理担当者は明日来るから、立ち会う。

Both are possible, but the first sounds more polite and suitable for formal conversation or explaining a situation calmly.

In customer-service or business-like contexts, ので is very common.

The subject is omitted because Japanese often leaves out things that are clear from context.

So 立ち会います here most naturally means:

• I will be present
• I will attend
• I will be there while it happens

Japanese frequently omits pronouns like:

• 私 = I
• あなた = you
• 彼 = he

If the context makes it obvious, they are simply not said.

So the sentence does not explicitly say I, but that is what is understood.

立ち会う means to be present at something, attend something in person, or be there as it happens.

In this sentence, it suggests that the speaker will be present when the repair person comes, probably to:

• let them in
• observe the repair
• assist if needed

It does not just mean stand in the physical sense. It is more like:

• be present for
• witness
• attend

So it is more specific than simply saying います.

Often 立ち会う is used with something like:

• に立ち会う = be present at / attend

For example:

• 修理に立ち会います。 = I will be present for the repair.

In your sentence, that part is omitted because it is already obvious from context: the speaker will be present for the repair person’s visit or the repair itself.

So the sentence is natural even without stating it explicitly.

The comma helps separate the reason clause from the main action:

• 修理担当者は明日来ますので、立ち会います。

It makes the sentence easier to read:

• reason: the repair person is coming tomorrow
• result: I will be present

Japanese commas are somewhat flexible, so you may sometimes see similar sentences without one. But here the comma is very natural and helps clarity.

Yes. A plain-style version would be something like:

• 修理担当者は明日来るので、立ち会う。

Changes:

• 来ます → 来る
• 立ち会います → 立ち会う

The meaning stays basically the same, but the tone becomes less polite and more casual.

So the original sentence is the polite version, which fits many real-life situations better.