Quando ho mal di stomaco, bevo il tè tiepido.

Because Italian uses avere for many physical conditions where English uses to be or to have in different ways.
Here, ho mal di stomaco literally follows the Italian pattern avere mal di + body part, meaning to have pain in ...

So:

• ho mal di stomaco = I have a stomachache / my stomach hurts
• ho mal di testa = I have a headache
• ho mal di gola = I have a sore throat

Using essere here would be incorrect.

Mal di stomaco is a fixed expression meaning stomachache or stomach pain.

It breaks down like this:

• mal = pain / ache / illness
• di = of
• stomaco = stomach

So literally it is something like pain of stomach, but in natural English we say a stomachache or stomach pain.

This pattern is very common in Italian:

• mal di testa = headache
• mal di denti = toothache
• mal di schiena = back pain

That is just how this idiomatic expression works in Italian. After avere, Italian normally says:

• avere mal di stomaco
• avere mal di testa
• avere mal di gola

So you do not usually say ho il mal di stomaco in standard everyday Italian.

By contrast, you do use an article in a different structure such as:

• ho un dolore allo stomaco = I have a pain in my stomach
• mi fa male lo stomaco = my stomach hurts

So the lack of article is tied to the idiom avere mal di ...

The present tense here expresses a habitual or general action:

• Quando ho mal di stomaco, bevo il tè tiepido.
  = Whenever I have a stomachache, I drink lukewarm tea.

It is not necessarily describing only one specific moment. It means this is what the speaker generally does in that situation.

Italian often uses the simple present this way, just like English can use the present for habits:

• When I have a stomachache, I drink lukewarm tea.

Italian often drops subject pronouns because the verb ending already tells you who the subject is.

Here:

• ho = I have
• bevo = I drink

So io is unnecessary unless you want emphasis or contrast.

Compare:

• Quando ho mal di stomaco, bevo il tè tiepido. = normal
• Quando io ho mal di stomaco, bevo il tè tiepido. = more emphatic, often unnecessary

This is one of the biggest differences from English, where the subject usually must be stated.

Here il tè tiepido means lukewarm tea as a noun phrase with a definite article.

Italian often uses the definite article in places where English may or may not use one. In this sentence, bevo il tè tiepido sounds natural as I drink lukewarm tea / I drink the lukewarm tea depending on context.

You might also hear:

• bevo tè tiepido

This is also possible, but it sounds a bit more like I drink lukewarm tea in a more general or substance-based sense.

So:

• bevo il tè tiepido = very natural, concrete, standard
• bevo tè tiepido = also possible, slightly less specific-sounding

No. Tiepido means lukewarm, slightly warm, or tepid. It is milder than caldo.

• caldo = hot / warm
• tiepido = lukewarm, gently warm, not hot

So il tè tiepido is tea that is warm but not very hot.

This adjective is often used for food, drinks, water, or even figurative situations.

The accent distinguishes tè meaning tea from te meaning you after a preposition.

Compare:

• tè = tea
• per te = for you

The accent mark helps avoid confusion. In writing, it is important to include it.

Yes, absolutely. Both are natural, but the structure is different.

• Ho mal di stomaco. = I have a stomachache.
• Mi fa male lo stomaco. = My stomach hurts.

The second one literally means the stomach causes pain to me.

Both are common. The first is a very compact idiomatic expression; the second focuses more directly on the body part itself.

Yes. That sentence is also correct.

Both are natural:

• Quando ho mal di stomaco, bevo il tè tiepido.
• Bevo il tè tiepido quando ho mal di stomaco.

The difference is mainly one of focus and rhythm:

• Starting with Quando ho mal di stomaco emphasizes the condition first.
• Starting with Bevo il tè tiepido emphasizes the action first.

When the quando clause comes first, a comma is commonly used, as in your sentence.

In this sentence, quando is best understood as when or whenever, depending on context.

Because the sentence describes a habit, English often naturally interprets it as:

• When I have a stomachache, I drink lukewarm tea.
• Whenever I have a stomachache, I drink lukewarm tea.

So even though Italian uses quando, the sense here is habitual, which often matches English whenever.