Learn Hardcore Japanese: tenkiyohou ni yoru to, raisyuu ha ame ni naru hazu da. - According to the weather forecast, it should become rainy next week.

It marks the source of information and means according to. It’s built from N + に + よる + と. The と is the quotative particle that introduces what follows as the content reported from that source. So 天気予報によると means according to the weather forecast.

Is the と in によると the conditional if?

No. Here と is the quotative particle (the same one used in 〜と言う), not the conditional if/when. It flags that what follows is the statement being attributed to the source.

How is によると different from によれば, では, or だと/って?

によると / によれば: Both mean according to; によると is a bit more common in everyday use.
では: As for/from the standpoint of. It can sound more like a general frame of reference (e.g., as for the weather forecast) rather than strictly cited information.
だと / って: Colloquial according to or they say; って is very casual.
All are possible with a subtle nuance difference. With published sources, によると/によれば feels most neutral/formal.

Why is it 来週は and not 来週に?

来週は uses は to set next week as the topic: as for next week.
来週に marks a specific point in time and is not natural with a broad weather statement like this. Use は (or omit the particle) for time-period topics: 来週、雨になるはずだ is also fine.

Why use 雨になる instead of 雨が降る or 雨だ?

雨になる: become rainy, highlights a change of state, which forecasts commonly talk about (e.g., it will turn to rain).
雨が降る: it will rain, describes the event of rain falling.
雨だ: it is rainy (noun predicate), states a condition.
All can be correct; forecasts frequently use 〜雨になるでしょう or 〜雨が降るでしょう. Choice depends on whether you emphasize the change, the event, or the state.

Can I say 来週は雨のはずだ?

Yes. With a noun predicate and はず, you use N + の + はず.

来週は雨のはずだ = It should be rainy next week (state).
来週は雨になるはずだ = It should become rainy next week (change).
Both are grammatical; pick the nuance you want.

What nuance does はずだ add, and how does it compare to でしょう, らしい, そうだ (hearsay), and に違いない?

はずだ: strong expectation based on reasoning or reliable info (here, the forecast).
でしょう: probabilistic, softer and more neutral; common in forecasts.
らしい: seems/appears so based on reports or signs; slightly vague.
そうだ (hearsay): I hear that; explicitly marks reported info (e.g., 雨が降るそうだ).
に違いない: must be; very strong conviction.
With a forecast as source, でしょう or hearsay そうだ are most typical; はずだ is okay and suggests the speaker’s confident inference from that source.

Is だ too casual at the end? How do I make it polite?

Yes, だ is plain. The polite form is です:
天気予報によると、来週は雨になるはずです。
Forecast-style politeness often uses 〜でしょう:
天気予報によると、来週は雨になるでしょう。

There are two に in the sentence. Are they the same?

They’re different uses of に:

In による, に attaches to the source noun to form according to.
In 雨になる, に marks the resulting state in the change-of-state pattern A は B になる.

Why is there no explicit subject like it?

Japanese often omits subjects when obvious from context. Weather expressions typically have no overt subject; 来週は sets the time as the topic, and the predicate describes the weather.

Can I change the word order?

Yes. These are all fine:

天気予報によると、来週は雨になるはずだ。
来週は、天気予報によると、雨になるはずだ。
来週、天気予報によると、雨になるはずだ。
Topic-first or source-first are both natural.

Is the comma after と required?

Not required, but common to mark a pause after the introductory according to phrase: 〜によると、〜. It improves readability.

Are there more natural-sounding forecast alternatives?

Common options include:

天気予報によると、来週は雨が降るそうだ。 (hearsay)
天気予報では、来週は雨になる見込みだ。 (outlook)
来週は雨の予報だ。 (concise)
天気予報だと、来週は雨らしい。 (colloquial)

Why are there spaces between words? Japanese usually doesn’t have them, right?

Correct. Standard Japanese writing doesn’t use spaces; they’re added here as a learner aid to show word boundaries. In native text you would write it without spaces.

How do I pronounce 雨 here, and is it the same as candy?

Both are read あめ, but the pitch accent differs in many dialects:

Rain (雨) is typically low–high.
Candy (飴) is typically high–low.
Context also disambiguates, and patterns vary by region.

How do I negate or change the tense of はずだ?

Negative expectation: 雨にはならないはずだ。 (It shouldn’t become rainy.)
Past expectation: 雨になるはずだった。 (It was supposed to become rainy.)
Strong denial: 雨になるはずがない。 (There’s no way it’ll become rainy.)