Learn Hardcore Japanese: netu ga sagaranakattara, asita haisya zyanakute byouin ni iku tumori da. - If my fever doesn’t go down, I plan to go to the hospital tomorrow instead of the dentist.

In Japanese, the 〜たら conditional always uses the た‑form of the verb, which looks like past tense but doesn’t necessarily mean past in this structure.

下がる → 下がったら = if/when (it) goes down
下がらない → 下がらなかったら = if/when (it) doesn’t go down

So 熱が下がらなかったら means “if my fever doesn’t go down” (in the future), not “if my fever didn’t go down (in the past)”.
The “past” form here is a grammatical requirement of the 〜たら pattern, not a past-time meaning.

What’s the difference between 熱が下がらなかったら and 熱が下がらなければ?

Both mean roughly “if the fever doesn’t go down,” but there are nuances:

〜なかったら
- Very common in everyday spoken Japanese
- Feels relatively neutral and natural in conversation
〜なければ
- Feels a bit more formal / written or “rule-like”
- Often used in patterns like 〜なければならない (“must ...”)

So:

熱が下がらなかったら、… = “If the fever doesn’t go down, (then) …” — casual, natural.
熱が下がらなければ、… = same basic meaning but slightly more formal/serious-sounding.

Why is が used after 熱 instead of は? Could you say 熱は下がらなかったら?

Here, 熱が marks 熱 as the grammatical subject of 下がる (“the fever goes down”).

In subordinate clauses like conditionals (〜たら, 〜とき, etc.), Japanese tends to use が for the subject.
は usually marks the topic and often adds a contrast or emphasis (“as for X…”).

You could say 熱は下がらなかったら, but it would sound more like:

“As for the fever, if it doesn’t go down, ...” which can feel a bit more contrastive or marked, and is less neutral than 熱が下がらなかったら.

What exactly is じゃなくて doing between 歯医者 and 病院?

じゃなくて is the casual te‑form of じゃない (“is not”), so literally:

歯医者じゃなくて → “not (a) dentist, and ... / but instead ...”

In this sentence:

明日 歯医者じゃなくて 病院に行くつもりだ
= “Tomorrow, not to the dentist but to the hospital, I plan to go.”

So じゃなくて connects 歯医者 (“dentist”) and 病院 (“hospital”), expressing “not A but B”.

A more formal version would be 歯医者ではなくて病院に….

Why is there no particle after 歯医者? Shouldn’t it be 歯医者に?

Logically, you might expect:

明日 歯医者にじゃなくて、病院に行くつもりだ

But in natural Japanese, when you say “not A but B” with じゃなくて, you usually:

Put じゃなくて right after the noun (歯医者),
Then only say the particle once, on the second item:

歯医者じゃなくて病院に行くつもりだ
= “I plan to go not to the dentist, but to the hospital.”

The に is understood to apply to both “歯医者” and “病院”, even though it is only said before “病院”.

How does つもりだ work here? What’s the nuance compared to just 行く or 行く予定だ?

つもり is a noun meaning “intention”. The pattern is:

[dictionary-form verb] + つもりだ
→ “to intend/plan to do [verb]”

So:

行くつもりだ = “I intend to go / I’m planning to go”

Nuance:

行く alone: simple statement about the future: “I will go / I’m going.”
行くつもりだ: emphasizes your personal intention/decision.
行く予定だ: more of a schedule/arrangement (“it’s scheduled that I’ll go”), feels a bit more external or formal.

Here, 行くつもりだ sounds like a personal decision:
“If the fever doesn’t go down, I’m planning (I’ve decided) to go to the hospital...”

Why does the sentence end with だ instead of です? Is this casual?

Yes. だ is the plain (informal) copula; です is the polite copula.

熱が下がらなかったら、明日歯医者じゃなくて病院に行くつもりだ。
→ Plain style; used with friends, family, etc.

Polite version:

熱が下がらなかったら、明日歯医者じゃなくて病院に行くつもりです。

The rest (行く, 下がらなかったら, じゃなくて) can stay in plain form even in polite speech, as long as the sentence ends in です/ます.

Why doesn’t 明日 have a particle like に? Is 明日 alone okay?

Yes, time expressions in Japanese often appear without any particle, especially in casual speech:

明日、学校に行く。 = “Tomorrow, (I’ll) go to school.”
昨日、映画を見た。 = “Yesterday, (I) watched a movie.”

You can say 明日に in some contexts, but:

明日病院に行く is the most natural way to say “I’ll go to the hospital tomorrow.”
明日に病院に行く is rare and usually feels off; に with pure time words is limited and somewhat specialized (e.g., 約束は明日にしましょう “Let’s set the appointment for tomorrow.”)

So here, 明日 with no particle is completely normal.

How do we know that the subject is “I” (that I will go), when there is no 私?

Japanese often omits pronouns when they’re obvious from context.

行くつもりだ (“intend to go”) almost always refers to the speaker’s own intention unless another subject is explicitly given.
If you wanted to say “He intends to go,” you’d normally say:
- 彼は明日歯医者じゃなくて病院に行くつもりだ。

Because no subject is stated, and we’re talking about the speaker’s own fever and plans, the natural default interpretation is “I intend to go.”

Could you say 歯医者じゃなくて病院に行く予定だ instead? How would that sound?

You can, and it’s grammatically fine:

熱が下がらなかったら、明日歯医者じゃなくて病院に行く予定だ。

Nuance:

行くつもりだ: “I intend to go” — emphasizes your own decision.
行く予定だ: “It is scheduled/planned that I will go” — sounds more like a set plan or appointment.

In everyday speech about your own possible action (especially conditional, “if the fever doesn’t go down”), つもりだ feels a bit more natural and personal than 予定だ.