Ihre Wange ist warm, weil sie Fieber hat.

Here, it is most naturally her.

Why this can be confusing:

• ihre can mean her
• Ihre can mean formal your

But because this word is at the beginning of the sentence, it has to be capitalized anyway, so the capital letter does not help much.

The second clause helps:

• weil sie Fieber hat = because she has a fever

So the sentence is clearly talking about she/her, not formal you. That makes Ihre Wange = her cheek.

Because Wange is a feminine noun: die Wange.

The possessive word ihr- changes its ending depending on the noun that follows it. Here the noun is:

• feminine
• singular
• nominative (the subject of the sentence)

So you get:

• ihr Arm = her arm
• ihre Wange = her cheek
• ihr Kind = her child

So ihre Wange is the correct form.

Because all nouns are capitalized in German.

So:

• die Wange
• das Fieber
• das Haus
• der Mann

This is a standard German spelling rule.

Because warm comes after ist and is being used as a predicate adjective.

In German, adjectives usually:

• do take endings before a noun
• do not take endings after verbs like sein, werden, or bleiben

So:

• ihre warme Wange = her warm cheek
• Ihre Wange ist warm = Her cheek is warm

That is why warm has no ending here.

Because weil is a subordinating conjunction meaning because.

In German subordinate clauses, the conjugated verb usually goes to the end.

So:

• Sie hat Fieber. = She has a fever.
• ..., weil sie Fieber hat. = ..., because she has a fever.

This is one of the most important word-order rules in German.

Because the part starting with weil is a subordinate clause, and in German that is normally separated by a comma.

So:

• Ihre Wange ist warm, weil sie Fieber hat.

That comma is required in standard German.

Because German usually says Fieber haben for to have a fever.

So the normal expression is:

• Sie hat Fieber.

Not:

• Sie hat ein Fieber. ❌

This is different from English, where you normally say a fever.

A useful way to remember it:

• English: have a fever
• German: Fieber haben

It refers to the person, not to Wange.

Why this question comes up:

• Wange is feminine
• sie can also refer to a feminine noun

So grammatically, you might wonder whether sie means the cheek. But the meaning makes that impossible, because a cheek does not have a fever. A person does.

So in this sentence:

• sie = she

Yes, often you can.

With denn, the meaning is also because, but the word order changes:

• Ihre Wange ist warm, weil sie Fieber hat.
• Ihre Wange ist warm, denn sie hat Fieber.

Difference:

• after weil, the verb goes to the end
• after denn, the clause keeps normal word order

Both are possible, but weil is very common in everyday German.

A simple pronunciation guide:

• Ihre ≈ EE-re
• Wange ≈ VAHNG-uh
• ist ≈ ist
• warm ≈ varm
• weil ≈ vile
• sie ≈ zee
• Fieber ≈ FEE-ber
• hat ≈ haht

A few helpful notes:

• w in German sounds like English v
• ie usually sounds like a long ee
• j is not in this sentence, but German j usually sounds like English y