onaka no tyousi ga warui toki ha, tumetai zyuusu wo nomanai hou ga ii.

お腹の調子 means the condition of your stomach or how your stomach feels.

• お腹 = stomach, belly
• 調子 = condition, state, how something is doing

So this is a very natural everyday way to talk about stomach trouble. It is less technical than talking about a specific medical organ.

Here, の links two nouns together.

• お腹 = stomach
• 調子 = condition

So お腹の調子 literally means the stomach’s condition or the condition of the stomach.

This use of の is extremely common in Japanese. It often works like of or ’s in English.

In お腹の調子が悪い, 調子 is the thing being described as 悪い.

So:

• 調子 = the condition
• が = marks it as the grammatical subject
• 悪い = bad

Together, 調子が悪い means the condition is bad.

This is a very common Japanese pattern:

• 気分が悪い = to feel sick
• 都合が悪い = to be inconvenient
• 調子が悪い = to be in bad condition / not feel well / malfunction

とき means time or when.

A plain-form phrase can come before とき to mean when... or at the time when...

So:

• お腹の調子が悪い = your stomach condition is bad
• お腹の調子が悪いとき = when your stomach condition is bad

Then は after とき marks that whole phrase as the topic:

• お腹の調子が悪いときは = as for when your stomach is not feeling well / when you have stomach trouble

So とき is a noun, and the whole clause before it modifies it.

The は marks お腹の調子が悪いとき as the topic of the sentence.

It gives the sense of:

• when that is the case...
• as for times when your stomach feels bad...

Without は, the sentence could still be understandable in some contexts, but ときは sounds very natural when giving general advice or setting up a condition.

Because Japanese adjectives normally come before the noun they modify.

So:

• 冷たいジュース = cold juice

This is just like:

• 大きい犬 = a big dog
• 新しい本 = a new book

If you say ジュースが冷たい, that means the juice is cold.
But here the sentence needs cold juice as the object, so 冷たい comes before ジュース.

Because ジュース is the direct object of 飲む.

• 飲む = to drink
• ジュースを飲む = to drink juice

So を marks the thing being drunk.

In this sentence:

• 冷たいジュースを飲まない = not drink cold juice

飲まないほうがいい literally means the not-drinking side is good, but naturally it means:

• you’d better not drink
• it’s better not to drink
• you shouldn’t drink

How it is built:

• 飲む = drink
• 飲まない = not drink
• ほうがいい = it is better to choose that option

So 飲まないほうがいい = it’s better not to drink

This is usually advice or a recommendation, not a hard command.

Compare:

• 飲まないほうがいい = you’d better not drink it
• 飲んではいけません = you must not drink it
• 飲まないでください = please don’t drink it

Also, ほう is often written in kana, though 方 is also possible.

The subject is omitted because it is understood from context.

Japanese often leaves out words like you, I, or people when they are obvious.

So this sentence is giving general advice to the listener, something like:

• When your stomach feels bad, don’t drink cold juice
• If you have an upset stomach, it’s better not to drink cold juice

Even though あなた does not appear, the meaning is still clear.

The comma is mainly there to make the sentence easier to read and to show a pause after the topic/condition:

• お腹の調子が悪いときは、

It is not carrying a special grammar meaning by itself. Japanese punctuation is often more flexible than English punctuation. The sentence would still be understandable without the comma, but the comma makes it clearer.

Sometimes yes, but the nuance changes.

• お腹 is everyday, natural, and broad. It can refer to the belly/stomach area in a casual way.
• 胃 means the actual stomach organ, so it sounds more specific or medical.

So:

• お腹の調子が悪い = my stomach feels bad / I have stomach trouble
• 胃の調子が悪い = my stomach organ is not in good condition

In ordinary conversation, お腹の調子が悪い is usually the more natural choice.

Literally, 悪い means bad, but in this expression it means something more natural like:

• not good
• upset
• unwell
• in bad condition

So お腹の調子が悪い does not mean your stomach is morally bad or anything like that. It means your stomach is not in good shape.

This is a good example of how Japanese often uses simple words like いい and 悪い in very natural everyday expressions.