Ohne Fieberthermometer weiß ich nicht, ob ich Fieber habe.

German often places a time, place, or condition phrase at the beginning of a sentence for emphasis or style.

Here, Ohne Fieberthermometer means without a thermometer, and it sets the condition for the whole sentence.

The basic idea is:

• Ohne Fieberthermometer = without a thermometer
• weiß ich nicht = I don’t know
• ob ich Fieber habe = whether I have a fever

Starting with Ohne Fieberthermometer puts the focus on the fact that not having a thermometer is the reason for the uncertainty.

Because German main clauses follow the verb-second rule.

That means the conjugated verb must come in the second position. If something other than the subject comes first, the verb still stays second, and the subject moves after it.

So:

• Normal order: Ich weiß nicht ...
• With a phrase moved to the front: Ohne Fieberthermometer weiß ich nicht ...

Structure:

• Ohne Fieberthermometer = position 1
• weiß = position 2
• ich = then the subject

This is very common in German.

Ohne always takes the accusative case.

So if there were an article, you would clearly see the accusative:

• ohne den Mantel = without the coat
• ohne einen Arzt = without a doctor

In your sentence, Fieberthermometer is a neuter noun, and because there is no article, the noun itself does not visibly change:

• ohne Fieberthermometer

So even though you do not see a different ending here, the preposition ohne still requires the accusative.

German often leaves out the article with nouns in a general or non-specific sense, especially after some prepositions.

Here, ohne Fieberthermometer means something like:

• without a thermometer
• without any thermometer available

It sounds natural and general.

You could also say:

• ohne ein Fieberthermometer

but that sounds more specific or marked, as if you are thinking of a particular thermometer or emphasizing the lack of one. In most everyday contexts, ohne Fieberthermometer sounds more natural.

Because German loves compound nouns.

Fieberthermometer is made from:

• Fieber = fever
• Thermometer = thermometer

Together, they form one noun meaning fever thermometer or simply thermometer in this context.

This is extremely normal in German:

• Krankenhaus = hospital
• Zahnbürste = toothbrush
• Wasserflasche = water bottle

In English, we often use separate words; in German, they are often written as one word.

Ob means whether or if in an indirect yes/no question.

The speaker does not know the answer to the question Do I have a fever? So German uses ob:

• Ich weiß nicht, ob ich Fieber habe.
• I don’t know whether I have a fever.

This is different from English if meaning a condition.

Compare:

• Ich weiß nicht, ob ich Fieber habe. = I don’t know whether I have a fever.
• Wenn ich Fieber habe, bleibe ich zu Hause. = If I have a fever, I stay home.

So here, ob is correct because the clause expresses uncertainty, not a condition.

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

So:

• Main clause: weiß ich nicht
• Subordinate clause: ob ich Fieber habe

In the subordinate clause:

• ob = clause marker
• ich = subject
• Fieber = object/complement
• habe = verb at the end

This is one of the most important sentence patterns in German.

Other examples:

• Ich weiß, dass er krank ist.
• Sie fragt, ob du kommst.

In German, many illness expressions use haben + noun without an article.

So you say:

• Ich habe Fieber. = I have a fever.
• Sie hat Hunger. = She is hungry.
• Er hat Angst. = He is afraid.

In English, a fever needs an article, but German normally just says Fieber with no article.

Ein Fieber is not the normal everyday expression here.

German distinguishes between wissen and kennen.

• wissen = to know a fact, information, or answer
• kennen = to know a person, place, or be familiar with something

Here the speaker means I do not know whether..., so this is knowledge of a fact. That requires wissen:

• Ich weiß nicht, ob ich Fieber habe.

You would use kennen in sentences like:

• Ich kenne den Arzt. = I know the doctor.
• Kennst du Berlin? = Do you know Berlin?

So wissen is the correct verb here.

That version is possible in theory, but it sounds less natural and can be harder to process.

The original sentence:

• Ohne Fieberthermometer weiß ich nicht, ob ich Fieber habe.

is clearer because the without a thermometer idea is established right at the start and obviously applies to the whole statement.

You could also say:

• Ich weiß ohne Fieberthermometer nicht, ob ich Fieber habe.

That is grammatical too, but it gives a slightly different emphasis. The original version is usually the most natural.

Not necessarily. Fieberthermometer is very clear and specific, but in everyday speech many people would simply say:

• Thermometer

So both can work depending on context:

• Ohne Thermometer weiß ich nicht, ob ich Fieber habe.
• Ohne Fieberthermometer weiß ich nicht, ob ich Fieber habe.

The longer compound just makes it extra explicit that it is a thermometer for checking fever.

Because in German, all nouns are capitalized.

So in this sentence:

• Fieberthermometer is a noun
• Fieber is also a noun

That is why both begin with capital letters.

This is a standard rule in German and helps learners spot nouns more easily.

Yes, it is natural and correct everyday German.

It sounds a little careful and complete, but absolutely normal. A native speaker might also say more casually:

• Ohne Thermometer weiß ich nicht, ob ich Fieber habe.
• Ich weiß nicht, ob ich Fieber habe — ich habe kein Thermometer.

But your original sentence is perfectly idiomatic and grammatically standard.