Learn Hardcore Japanese: yoru ni kawa de hanabi wo miru yotei da. - I plan to watch fireworks by the river at night.

The particle に marks a specific point in time—“at night.” It tells you when the action happens. With some time words (like 夜, 今日, 明日), you can actually drop に in casual contexts (“夜、川で…”) and still be understood. However, using に is more standard and often clearer, especially in writing or more formal speech.

Why do we use the particle で after 川, instead of に or another particle?

The particle で marks the location where an action takes place. Here, “viewing fireworks” happens at the river, so we say 川で. Using に with 川 (川に) would imply “to” the river or “being located” at the river (川にいる), but not “doing” something there.

Why is 花火 marked with を?

を indicates the direct object of the verb 見る (“to see/watch”). Since fireworks are what you’re watching, you mark 花火 with を. If you used が, as in 花火が見える, it would mean “fireworks are visible” (an intransitive nuance), not “I will watch the fireworks.”

Why does the verb 見る appear in its dictionary (plain) form before 予定?

In Japanese, you attach the plain-dictionary form of a verb directly to 予定 to form “plan to do ~.” So 見る modifies 予定 just like any adjective/relative clause modifies a noun (e.g. 来る人, 食べる時間). The pattern is:
【Verb-dictionary form】＋予定だ → “it’s planned to [verb]”

Why is there no の between 見る and 予定?

When a verb in plain form modifies a noun, it acts like a relative clause—you don’t need the nominalizer の. You say 見る予定, not 見るの予定. (By contrast, if you wanted to turn a verb into a noun phrase by itself—e.g. “watching” as a concept—you’d use 見るの.)

What nuance does 予定 add compared to, say, つもり or other ways to express plans?

予定 (plan/schedule) often implies an arrangement or schedule that may be set by someone or some group, or at least is fairly established.
つもり (intention) expresses personal will or determination.
Example:
明日行く予定だ。 → “It’s scheduled/I have a plan to go tomorrow.”
明日行くつもりだ。 → “I intend to go tomorrow.”
Use 予定 when you talk about a set schedule and つもり when you emphasize your own intention.

Why does the sentence end with the copula だ instead of です?

だ is the plain (informal) copula. です is its polite counterpart. In casual or written-neutral speech you’ll see だ. If you want to be more polite, switch to 予定です.

Why is there no explicit subject (like 私は) in this sentence?

Japanese often omits the subject when it’s clear from context. Here, it’s understood that “I” am the one who plans to watch fireworks. Adding 私は is possible for emphasis (“As for me, I plan…”), but it’s not needed if the speaker is obvious.

Why is the time expression 夜に placed before the location 川で? Can you swap them?

The common Japanese order for adverbials is: Time → Location → Object → Verb. Putting 夜に first and 川で next follows that pattern. You could say 川で夜に花火を見る予定だ, but it sounds slightly awkward—time usually comes before place. Swapping is grammatically allowed but breaks the most natural rhythm.

Could you omit the particle に after 夜 to say simply 夜? What changes?

Yes. In casual speech you can drop に:
夜、川で花火を見る予定だ。
It still means “At night…” but feels more colloquial. Note you cannot drop で after 川 (that would leave no marker for “at the river”), so only the time particle is optional here.