Wenn das Fieberthermometer wieder verschwunden ist, frage ich unsere Krankenkasse, ob sie ein neues bezahlt.

Here wenn can be understood as if or when, depending on context. It is commonly used for:

• conditions: if
• repeated or general situations in the present/future: when/whenever

In this sentence, it suggests something like if/when the thermometer has disappeared again. German uses wenn for this kind of future or repeatable situation. It would not be als, because als is used for a one-time event in the past.

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

So:

• main clause word order: verb usually in second position
• subordinate clause word order: finite verb goes to the end

That is why you get:

• Wenn ... verschwunden ist not
• Wenn ... ist verschwunden

The verb verschwinden forms its perfect tense with sein, not haben.

So:

• Das Thermometer ist verschwunden. = the thermometer has disappeared

Many intransitive verbs involving movement or a change of state use sein in the perfect tense, and verschwinden belongs to that group.

It is the past participle of verschwinden, and together with ist it forms the perfect tense:

• ist verschwunden = has disappeared

At the same time, verschwunden can also feel adjective-like in meaning, because it describes the result: the thermometer is gone. But grammatically here it is best understood as part of the perfect tense.

Because the sentence begins with a subordinate clause:

• Wenn das Fieberthermometer wieder verschwunden ist, ...

After that whole clause, the main clause begins. In German main clauses, the finite verb must still be in position 2. The subordinate clause counts as position 1, so the verb comes next:

• ..., frage ich unsere Krankenkasse ...

This is normal German word order after an introductory clause.

Because fragen takes a direct object in the accusative case.

Here:

• ich frage wen? → unsere Krankenkasse

So unsere Krankenkasse is accusative singular.

Ob introduces an indirect yes/no question, and in English it often corresponds to whether or sometimes if.

So:

• Ich frage unsere Krankenkasse, ob sie ein neues bezahlt. means
• I ask our health insurance whether it pays for a new one.

This is not the same if as a condition. In German, conditional if is usually wenn, while indirect whether/if is ob.

For the same reason that ist is at the end of the wenn clause: ob also introduces a subordinate clause, and subordinate clauses send the finite verb to the end.

So:

• ..., ob sie ein neues bezahlt

That word order is required in standard German.

Sie refers to die Krankenkasse.

Even though a health insurance company is not a female person, the noun Krankenkasse is grammatically feminine, so the pronoun is sie.

German pronouns follow grammatical gender:

• die Krankenkasse → sie

Because German can leave the noun out when it is obvious from context, just like English can say a new one.

So:

• ein neues = a new one
• understood full version: ein neues Fieberthermometer

This is very natural German.

Because Fieberthermometer is neuter: das Fieberthermometer.

If the full phrase were written out, it would be:

• ein neues Fieberthermometer

When the noun is omitted, the adjective keeps the ending that matches the missing noun. So neues shows that the missing word is neuter singular.

Wieder means again. It shows that this is not the first time the thermometer has disappeared.

So the sentence implies something like:

• it has gone missing before
• now it has gone missing again

Without wieder, the sentence would just say that it has disappeared, with no idea of repetition.

The perfect tense in the wenn clause shows that the disappearing happens first and is complete:

• wenn ... verschwunden ist = once it has disappeared / once it is gone

The main clause frage ich is present tense, which in German can describe a present habit, a general procedure, or even a future action.

So the sentence structure is roughly:

• once that has happened, I ask the insurance

That combination is very normal in German.

Because German normally uses commas to separate subordinate clauses from main clauses.

Here there are two subordinate clauses:

• Wenn das Fieberthermometer wieder verschwunden ist
• ob sie ein neues bezahlt

So the commas are required:

• after the wenn clause
• before the ob clause

German comma rules are generally stricter than English ones in this area.