Learn Hardcore Japanese: asita ha kaigisitu de douryou no tonari ni suwarimasu. - Tomorrow I will sit next to my coworker in the meeting room.

は marks the topic: it frames “as for tomorrow” and can add a mild contrast with other times (not yesterday, not today).
Both are fine:
- 明日は会議室で… sets tomorrow as the theme.
- 明日、会議室で… simply states a time adverb; it feels a bit more neutral.
Use は if you want to contrast or keep “tomorrow” as the ongoing topic in the conversation.

Can I say 明日に? When do I use に with time expressions?

With adverbs like 今日, 明日, 昨日, 毎日, you normally omit に: 明日行きます.
に is used with specific points in time: 3時に, 月曜日に, 1月1日に.
明日に is generally unnatural in this kind of sentence. It can appear in set phrases or special nuances:
- 明日にする “let’s make it tomorrow” (deciding a deadline/appointment)
- 明日には… to add contrast/emphasis or “by tomorrow at least”

Why is 会議室 marked with で and not に?

で marks the place where an action occurs: 会議室で話す “talk in the conference room.”
に marks destination or existence: 会議室に行く “go to the conference room”; 会議室にいる “be in the conference room.”
With 座る, に marks the target surface/spot (椅子に座る “sit on a chair”), while で marks the broader location of the action (会議室で座る “do the act of sitting in the conference room”). Your sentence is correct.

Why is it 同僚のとなり and not 同僚となり?

の links nouns. X のとなり means “the next-to-X position,” literally “X’s next (spot).”
となり is a noun, so it needs の to attach to another noun: 同僚のとなり, 先生のとなり, 駅のとなり, etc.

Why is it となりに座ります and not となりで座ります?

With positional nouns (となり, まえ, うしろ, となり, 中, 上, 下), use に to mark the exact position/goal for placement or existence verbs: となりに座る, まえに立つ, 机の上にある.
で with となり can be used with other actions (“to do something while at/around that spot”): 友達のとなりで話す. But for “sit,” use となりに座る.

What’s the nuance difference between となり, よこ, and そば/近く?

となり(隣): “adjacent/next,” typically immediately next with nothing in between; often used for same-type items (seats, rooms, houses) but also fine with people. Strong “right next to” feel.
よこ(横): “beside/to the side of.” More about lateral position; also used for the side part of an object (本棚の横).
そば/近く: “near/close to,” not necessarily adjacent.
In your sentence, 同僚のとなりに座ります emphasizes an immediately adjacent seat.

Is it okay to write 隣 instead of となり?

Yes. 隣 is the standard kanji; となり is the reading. Both are fine: 同僚の隣に座ります = 同僚のとなりに座ります.
Beginners often see となり in hiragana; everyday writing often uses the kanji 隣.

Do I need to say 私の同僚 to mean “my coworker”?

Usually no. Context makes it obvious you mean your coworker.
Add a possessor only if needed: 私の同僚, 彼の同僚, うちの同僚 (natural, casual/in-house nuance).

Should I add さん to 同僚?

Not to the role noun alone. Say the person’s name + さん: 同僚の田中さん.
同僚さん is sometimes heard in service speech referring to someone else’s coworker, but it can sound odd in ordinary conversation.

Why does 座ります (non-past) mean “will sit”? How do I show future meaning?

Japanese has no dedicated future tense. The non-past (-る/-ます) covers present and future; time words and context supply the future meaning.
To express intention or plan:
- 明日は同僚のとなりに座るつもりです (intend to)
- 明日は同僚のとなりに座る予定です (scheduled/plan)
- 明日は同僚のとなりに座ると思います (I think I will)

What’s the difference between 座ります and 座っています?

座ります: the action “sit (down).”
座っています: the state “be seated.” For a predicted future state, add context: 明日の会議中は同僚のとなりに座っていると思います.

How do I make this casual, past, or negative?

Can I change the word order?

Japanese word order is flexible as long as particles stay attached, but the neutral flow is Time → Place → Position/Goal → Verb:
- 明日は会議室で同僚のとなりに座ります.
Other orders add emphasis/contrast:
- 会議室では、明日、同僚のとなりに座ります (contrasts the place)
Avoid scattering too wildly; keep it easy to parse.

Why is there no explicit subject? How would I say “I”?

Subjects are often omitted when obvious. Here, it’s likely “I.”
To state it: 私は明日、会議室で同僚のとなりに座ります. Usually pick one topic (私は or 明日は); using both can be okay when contrasting multiple things, but don’t overuse.

Could I say 会議室に instead of 会議室で here?

Not with this meaning. に with 座る marks the specific thing you sit on/in: 椅子に座る, 床に座る.
会議室 is a place, not a seat. Say 会議室で (action location) or specify a seat: 会議室の椅子に座る.

How do I pronounce each word?

Can I make the seat explicit?

Yes: 同僚の隣の席に座ります (“I’ll sit in the seat next to my coworker”). This removes any ambiguity and sounds very natural for seating contexts.

Is it normal to have spaces between the words like in the given sentence?

In real Japanese writing, spaces are generally not used. They were added for learners. The natural sentence is:
- 明日は会議室で同僚のとなりに座ります.

What are common mistakes to avoid with this pattern?

Using となりで座る (should be となりに座る).
Saying 明日に行きます in this context (say 明日行きます).
Using 会議室に座る to mean “sit in the conference room” (prefer 会議室で… or specify a seat).
Saying 同僚さん instead of 同僚の田中さん (attach さん to names, not the role noun).