Wenn mir schwindelig ist, setze ich mich hin und trinke Wasser.

In German, schwindelig sein is commonly used with a dative experiencer:

• Mir ist schwindelig. = I feel dizzy.
• Literally, it is closer to To me, it is dizzy than to I am dizzy.

So:

• mir = dative form of ich
• schwindelig = dizzy
• ist = is

This is just the normal German pattern for this expression. German often uses the dative with physical or emotional states like this.

Because wenn introduces a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• Wenn mir schwindelig ist
  literally: When to me dizzy is

Compare:

• Main clause: Mir ist schwindelig.
• Subordinate clause: wenn mir schwindelig ist

This verb-final word order is one of the most important patterns in German.

Here, wenn means when or whenever.

It is often used for:

• repeated situations
• general conditions
• things that happen whenever a certain situation occurs

So this sentence means something like:

• When/Whenever I feel dizzy, I sit down and drink water.

It is not a one-time past event. If it were a one-time event in the past, German would usually use als instead of wenn.

Because in German, a subordinate clause is normally separated from the main clause by a comma.

So:

• Wenn mir schwindelig ist, = subordinate clause
• setze ich mich hin und trinke Wasser. = main clause

This comma is required in standard German.

Because when a sentence begins with a subordinate clause, the main clause still follows the normal German verb-second rule.

The first position in the whole sentence is already occupied by:

• Wenn mir schwindelig ist

So in the main clause, the conjugated verb must come next:

• setze = second position
• ich = subject after the verb

That is why you get:

• Wenn mir schwindelig ist, setze ich mich hin.

not

• Wenn mir schwindelig ist, ich setze mich hin.

Because sich hinsetzen is a reflexive verb in German.

The pattern is:

• sich hinsetzen = to sit down

So you say:

• ich setze mich hin
• du setzt dich hin
• er setzt sich hin

Here, mich is the reflexive pronoun matching ich.

Also, in this expression the reflexive pronoun is in the accusative:

• mich
• dich
• sich

Hinsetzen is a separable verb:

• sich hinsetzen = to sit down

In a normal main clause, separable prefixes split off and go to the end:

• Ich setze mich hin.

So:

• setze = main verb part
• hin = separable prefix

The hin adds the idea of movement to a position. Here it helps express sitting down, not just being seated.

They are different in meaning:

• ich setze mich hin = I sit down
  → movement into a seated position
• ich sitze = I am sitting
  → state, already seated

So in this sentence, setze ich mich hin is correct because it describes the action you take when you feel dizzy.

Because the same subject, ich, applies to both verbs:

• setze ich mich hin
• und trinke Wasser

In English, we often repeat the subject, but in German it is very common not to repeat it when the subject stays the same.

So this is completely natural:

• ..., setze ich mich hin und trinke Wasser.

You could repeat ich, but here it would usually sound unnecessary.

Because Wasser here is being used as a mass noun in a general sense.

German often omits the article with substances when talking about them in a general, non-specific way:

• Ich trinke Wasser. = I drink water.
• Sie kauft Brot. = She buys bread.

If you said das Wasser, it would usually sound more specific, like the water.

Yes. Schwindelig is an adjective, and here it is used with sein as a predicate adjective:

• Mir ist schwindelig.

So it is not acting like a noun, and it does not need an ending here. This is similar to:

• Mir ist kalt.
• Mir ist warm.
• Mir ist schlecht.

These are very common German patterns for describing physical sensations.

Yes, you could add dann if you want to make the sequence more explicit:

• Wenn mir schwindelig ist, dann setze ich mich hin und trinke Wasser.

This is grammatical and understandable. But dann is not required. Without it, the sentence sounds perfectly natural and a bit simpler.