Learn Hardcore German: Wenn mir übel ist, bleibe ich im Bett und lese ein Buch. - When I feel sick, I stay in bed and read a book.

The feeling (übel) is what “is,” and you are the person to whom this state applies, so you appear in the dative (mir), not as the subject (ich).

Why is it „mir“ and not „mich“?

mir = dative (to me, for me)
mich = accusative (me as a direct object)

Expressions for physical or emotional states in German often use the dative:

Mir ist kalt. – I’m cold.
Mir ist langweilig. – I’m bored.
Mir ist übel. – I feel sick.

So „Wenn mir übel ist“ literally means “When it is sick to me,” which is just how German structures this kind of feeling.

Could you also say „Wenn es mir übel ist“? Where is the „es“?

The basic pattern can be:

Es ist mir übel.
Mir ist übel. (the es is dropped)

Both forms exist. The „es“ here is just a dummy subject (like “it” in “it is raining”).

In subordinate clauses (with wenn, weil, dass, etc.), German very often drops this dummy es, especially when the verb goes to the end:

wenn (es) mir übel ist → normally said as „wenn mir übel ist“

So „Wenn mir übel ist“ is the natural, idiomatic form.
„Wenn es mir übel ist“ is grammatically possible, but feels heavier and is rarely used in everyday speech.

Why does the verb „ist“ go to the end in „Wenn mir übel ist“?

Wenn introduces a subordinate clause (Nebensatz).
In German subordinate clauses, the finite verb goes to the very end:

wenn
- [subject/objects/etc.] + verb (at the end)

So:

Wenn mir übel ist → wenn (subordinating conjunction) → mir übel → ist (verb at the end)

This is a core word-order rule:
Subordinate clause → conjugated verb at the end.

Why is it „bleibe ich im Bett“ and not „ich bleibe im Bett“?

In a main clause (Hauptsatz), German is verb-second (V2):

Exactly one element comes before the finite verb.
The verb is then in second position.
The subject is not required to be first.

In your sentence, the entire subordinate clause counts as one element in front of the main clause:

[Wenn mir übel ist], bleibe ich im Bett …
- Element 1: the whole clause „Wenn mir übel ist“
- Element 2: the verb bleibe
- Then the subject: ich

If you remove the wenn-clause, the normal order is:

Ich bleibe im Bett.

But once you put „Wenn mir übel ist“ first, German grammar requires „bleibe ich …“, not „ich bleibe …“.

What is „im Bett“ exactly? Why not just „in Bett“?

„Im“ is a contraction of „in dem“:

in + dem Bett → im Bett

The preposition in can take:

dative (location, “in” somewhere, no movement)
accusative (direction, into somewhere, with movement)

Here it’s a location (you’re lying in bed, not moving there), so dative is used:

Wo bin/bleibe ich? – im Bett. (dative: dem Bett)

So:

im Bett = in dem Bett = “in bed.”
Saying „in Bett“ without an article is not correct in this context.

Why „bleibe ich im Bett“ instead of „bin ich im Bett“?

Both are grammatical, but the meanings differ:

Ich bin im Bett. – I am in bed. (just states your location)
Ich bleibe im Bett. – I stay in bed. (emphasizes that you do not get up)

Your sentence talks about what you do when you feel sick: you stay in bed (you don’t leave it). That’s why „bleibe“ is the better, more natural verb here.

Why is it „und lese ein Buch“ and not „und ich lese ein Buch“?

Both are correct:

… bleibe ich im Bett und lese ein Buch.
… bleibe ich im Bett und ich lese ein Buch.

In German, when two main clauses share the same subject (ich), the second „ich“ can be omitted after und:

Ich stehe auf und gehe ins Bad.
(instead of Ich stehe auf und ich gehe ins Bad.)

The version without the repeated subject („und lese“) is more natural and stylistically smoother here.

Why is the comma there: „Wenn mir übel ist, bleibe ich …“?

German requires a comma between:

a subordinate clause (Nebensatz) and
the main clause (Hauptsatz).

Here:

Subordinate clause: Wenn mir übel ist
Main clause: bleibe ich im Bett und lese ein Buch

So a comma must separate them:

Wenn mir übel ist, bleibe ich im Bett …

If you reversed the order, you’d still use a comma:

Ich bleibe im Bett, wenn mir übel ist.

Why is „wenn“ used here and not „als“ or „falls“?

The three words have different uses:

wenn
- “when(ever)” in the sense of whenever / every time that
- also “if” in conditional sentences
- used for repeated or general situations (all tenses)
als
- “when” for a single event in the past
- e.g. Als ich krank war, blieb ich im Bett. – “When I was ill (that one time), I stayed in bed.”
falls
- more like “if / in case”
- emphasizes possibility / condition, often more hypothetical

Your sentence describes a general, repeated situation (a habit: whenever I feel sick, I do X), so „wenn“ is correct:

Wenn mir übel ist, bleibe ich im Bett …
= “Whenever I feel sick, I stay in bed …”

What exactly does „übel“ mean here? How is it different from „schlecht“ or „krank“?

In this context, „übel“ usually means:

feeling nauseous, queasy, or generally physically unwell.

Compare:

Mir ist übel. – I feel nauseous / I feel sick (in the stomach).
Mir geht es schlecht. – I feel bad / I’m doing poorly (could be physical or emotional).
Ich bin krank. – I am ill / I am sick (I have an illness).

So:

„Mir ist übel“ is a symptom: “I feel nauseous / queasy.”
„Ich bin krank“ is a state: “I am ill.”

In your sentence, „Wenn mir übel ist“ focuses on the sick feeling, not necessarily on having a diagnosed illness.

Why is „Bett“ capitalized, but „übel“ and „krank“ (if used) are not?

In German:

All nouns are capitalized.
Adjectives, verbs, pronouns, etc. are not capitalized (except at the beginning of a sentence or in some special cases).

Here:

Bett is a noun → capitalized.
übel is an adjective used predicatively → lowercase.
bleibe, lese are verbs → lowercase.
Wenn is capitalized only because it is the first word of the sentence.

So the capitalization in „Wenn mir übel ist, bleibe ich im Bett und lese ein Buch.“ follows the regular German rules.