ooame no toki ha, kawa no tikaku ni ikanai hou ga ii desu.

Because 大雨 and とき are both nouns.

• 大雨 = heavy rain
• とき = time

So 大雨のとき literally means the time of heavy rain. In more natural English, that becomes when there is heavy rain or when it rains heavily.

This is a very common Japanese pattern: noun + の + noun.

Examples:

• 夏の休み = summer vacation
• 試験の前 = before the test
• 雨の日 = rainy day

とき means time or when, and は marks that whole time phrase as the topic.

So:

• 大雨のとき = when there is heavy rain
• 大雨のときは = as for when there is heavy rain...

The は helps set the condition or situation for the rest of the sentence. It has a slightly rule-like or general advice feel:

• 大雨のときは、... = When there is heavy rain, ...

Without は, the sentence could still be grammatical in some contexts, but は is very natural when giving general guidance.

Japanese often uses a noun phrase where English uses a full clause.

So instead of saying something like when heavy rain falls, Japanese naturally says at the time of heavy rain:

• 大雨のとき = at the time of heavy rain

This is shorter and very natural in Japanese. It does not sound incomplete.

If you wanted a full verb clause, you could say something longer, such as:

• 雨がたくさん降っているとき

But 大雨のとき is much more natural and efficient here.

近く means near, nearby area, or vicinity.

In this sentence, it behaves like a noun:

• 川の近く = the area near the river

So the structure is:

• 川 = river
• の = linking particle
• 近く = vicinity / nearby area

Together: the river’s nearby area → near the river

You can use 近く with many nouns:

• 駅の近く = near the station
• 家の近く = near the house
• 学校の近く = near the school

Because 行く is a verb of movement, and に marks the destination or target location.

• 川の近くに行く = go near the river

Here, に tells you where someone goes.

Compare:

• 川の近くに行く = go to a place near the river
• 川の近くで遊ぶ = play near the river

So:

• に = destination
• で = place where an action happens

That is why に is the right choice with 行く here.

Sometimes へ can replace に with 行く, but に is more natural here.

• 川の近くに行く = go to the area near the river
• 川の近くへ行く = go toward the area near the river

へ emphasizes direction a bit more, while に sounds more like the destination itself. In this sentence, に is the most standard choice.

Because the pattern ～ほうがいいです uses the plain form before it.

So you say:

• 行かないほうがいいです not
• 行きませんほうがいいです ✗

This is very common in Japanese: the verb inside the grammar pattern is plain, and the politeness comes at the end with です.

Other examples:

• 早く寝たほうがいいです = You should go to bed early.
• 無理しないほうがいいです = You shouldn’t overdo it.

This is a very common advice pattern:

• plain negative verb + ほうがいい

So:

• 行かない = not go
• ほうがいい = is better

Together:

• 行かないほうがいい = it’s better not to go / you shouldn’t go

A useful thing to remember is that this expression often sounds much more natural in English as shouldn’t or had better not, even though the Japanese literally looks like the not-going side is better.

Also note a common contrast:

• 行ったほうがいい = you should go
• 行かないほうがいい = you shouldn’t go

For positive advice, Japanese usually uses the past plain form before ほうがいい:

• 行くほうがいい is possible in some contexts, but 行ったほうがいい is much more common for advice.

Because ～ほうがいい is basically a fixed expression.

Historically and grammatically, ほう means something like side, option, or choice, and が marks it as the thing being judged by いい.

So the literal idea is:

• The option of not going is good/better

In practice, learners should usually memorize ～ほうがいい as one chunk meaning:

• should
• had better
• it’s better to...

It is mainly advice, not a direct command.

• 行かないほうがいいです = You shouldn’t go / It’s better not to go
• softer than a strict prohibition

Stronger alternatives would be:

• 川の近くに行ってはいけません。 = You must not go near the river.
• 川の近くに行くな。 = Don’t go near the river.
  This is very strong and blunt.

So the original sentence sounds like a polite safety warning or recommendation.

Japanese often leaves out the subject when it is obvious or general.

In this sentence, the omitted subject could mean:

• you
• people
• one
• anyone

So the sentence is giving general advice, not necessarily talking about one specific named person.

This is very normal in Japanese. English usually needs a subject, but Japanese often does not.

Because in grammar patterns like ～ほうがいい, it is very often written in hiragana.

Both are possible:

• 行かないほうがいい
• 行かない方がいい

But hiragana is extremely common because ほう in this pattern functions more like a grammar word than a full lexical noun.

So if you see it in hiragana, that is completely normal.

No. In normal Japanese writing, you would usually write it without spaces:

大雨のときは、川の近くに行かないほうがいいです。

The spaces are often added only in teaching materials to make the sentence easier for learners to read and break into parts.