Ich habe heute Fieber, deshalb bleibe ich im Bett.

Because in German, the normal expression is Fieber haben = to have a fever.

German often uses haben for illnesses or symptoms:

• Ich habe Fieber.
• Ich habe Kopfschmerzen.
• Ich habe eine Erkältung.

So even though English sometimes says I am sick, for fever specifically German says I have fever / a fever.

In German, Fieber is usually used without an article in this expression:

• Ich habe Fieber.

This is just the standard idiomatic way to say it. English usually says a fever, but German normally does not say ein Fieber here.

Deshalb means therefore, that’s why, or for that reason.

It connects the first idea to the result:

• Ich habe heute Fieber = I have a fever today
• deshalb bleibe ich im Bett = therefore / that’s why I’m staying in bed

Other common words with a similar meaning are:

• deswegen
• darum

Because deshalb takes the first position in the clause, and in a German main clause the finite verb must come second.

So the order is:

• Deshalb = position 1
• bleibe = position 2
• ich = then comes the subject

That is why you get:

• Deshalb bleibe ich im Bett.

This is one of the most important German word-order rules: the verb is in second position in main clauses.

Because this sentence contains two main clauses:

• Ich habe heute Fieber
• deshalb bleibe ich im Bett

In German, it is common and correct to separate such clauses with a comma. You could also write them as two separate sentences:

• Ich habe heute Fieber. Deshalb bleibe ich im Bett.

Both are fine.

Bleibe is the 1st person singular present tense of bleiben = to stay / remain.

The present tense forms are:

• ich bleibe
• du bleibst
• er/sie/es bleibt
• wir bleiben
• ihr bleibt
• sie/Sie bleiben

So bleibe matches ich.

Because Bett needs an article here. Im is a contraction of:

• in dem → im

So:

• im Bett = in the bed / in bed

In German, this is the normal expression for staying in bed.

Because im Bett describes location, while ins Bett describes movement toward a destination.

• im Bett bleiben = to stay in bed
• ins Bett gehen = to go to bed

So in your sentence, the person is already describing where they will remain, not where they are going:

• deshalb bleibe ich im Bett

Because after in, German uses:

• dative for location
• accusative for direction/movement toward something

Here, bleiben shows location, not movement:

• ich bleibe im Bett → where? in bed → dative

Compare:

• Ich bin im Bett. → location
• Ich gehe ins Bett. → movement into bed

Heute means today. It gives a time reference.

In your sentence:

• Ich habe heute Fieber.

That is a very natural position. But German is flexible, and you could also say:

• Heute habe ich Fieber.

Both are correct. The difference is mainly emphasis:

• Ich habe heute Fieber = neutral
• Heute habe ich Fieber = stronger focus on today

Yes, that is possible.

• deshalb = therefore / for that reason
• also = so

But there is a grammar difference:

With deshalb, the verb comes second after it:

• Deshalb bleibe ich im Bett.

With also used as a conjunction in everyday speech, you often get:

• ..., also bleibe ich im Bett.

So both work, but deshalb is often a little clearer for learners because it strongly shows the result relationship.

Yes, bleiben is very natural here because it means stay:

• deshalb bleibe ich im Bett = that’s why I’m staying in bed

You could also say:

• deshalb liege ich im Bett

But that means more literally:

• that’s why I’m lying in bed

So:

• bleiben focuses on remaining there
• liegen focuses on the physical position

Not perfectly.

A very literal version would be:

• I have today fever, therefore stay I in the bed.

But natural English would be:

• I have a fever today, so I’m staying in bed.

German and English often use different word order and different article patterns, so it is better to learn the German sentence as a normal German structure rather than translating each word mechanically.