Learn Hardcore German: Dem Mann ist übel, weil er zu schnell gegessen hat. - The man feels sick because he ate too fast.

Mir ist kalt. – I am cold. (literally: To me is cold.)
Ihr ist langweilig. – She is bored. (literally: To her is boring.)
Dem Mann ist übel. – The man feels sick. (literally: To the man is nauseous.)

So dem Mann is in the dative because the pattern is:

jemandem ist übel / kalt / langweilig / schlecht …
(to someone is nauseous / cold / bored / sick …)

If you changed it to Der Mann ist übel, that would be nominative and it would mean something quite different (see next question).

What would Der Mann ist übel mean, compared to Dem Mann ist übel?

These two sentences have very different meanings:

Dem Mann ist übel.
- Case: dative (dem Mann)
- Meaning: The man feels sick / nauseous.
Der Mann ist übel.
- Case: nominative (der Mann)
- Meaning: The man is bad / nasty / evil (morally, as a person), not physically sick.

So the choice of dative (dem) vs nominative (der) completely changes the meaning here.

If dem Mann is dative, then what is the grammatical subject of Dem Mann ist übel? There is no es.

Grammatically, this is an impersonal construction. The underlying full version is:

Es ist dem Mann übel. – It is nauseous to the man.

Here, es is a dummy subject (like it in English in It is raining.). In everyday German, this es is very often dropped, especially in short, common expressions:

Es ist mir kalt. → Mir ist kalt.
Es ist ihr schlecht. → Ihr ist schlecht.
Es ist dem Mann übel. → Dem Mann ist übel.

So:

Logical subject: dummy es (often omitted)
Experiencer: dem Mann in the dative
Predicate adjective: übel

Even without es, native speakers feel ist übel as a complete predicate in these patterns.

Can I also say Es ist dem Mann übel or Dem Mann ist es übel? Are they correct?

Yes, both are grammatically correct:

Es ist dem Mann übel. – fully explicit, with the dummy es as subject.
Dem Mann ist es übel. – also possible; here the es is not clearly a dummy anymore, but many speakers accept this word order in speech.

However, the most natural everyday version is simply:

Dem Mann ist übel.

That is the most common spoken form.

What exactly does übel mean here, and how is it different from schlecht, krank, etc.?

In this construction, übel describes a nauseous, queasy feeling, often in the stomach:

Mir ist übel. – I feel nauseous / I feel sick to my stomach.

Comparisons:

Mir ist übel. – I feel nauseous (stomach upset, about to vomit).
Mir ist schlecht. – Very similar in everyday speech; many speakers use it the same way as übel.
Ich bin krank. – I am ill (more general: flu, infection, etc.).
Mir ist schwindlig. – I feel dizzy.
Ich fühle mich nicht wohl. – I don’t feel well.

Also, outside this set phrase, übel can mean bad / nasty / evil, especially about character or actions:

Er ist ein übler Mensch. – He is a nasty / bad person.
Das ist übel. – That’s bad / nasty.

In Dem Mann ist übel, though, the meaning is clearly physical nausea.

What does zu mean in zu schnell gegessen? Can I say sehr schnell gegessen instead?

In this sentence:

zu schnell means too fast (excessive, more than is good).

So:

weil er zu schnell gegessen hat – because he ate too fast.

Compare:

Er hat sehr schnell gegessen. – He ate very fast (just describing speed, not necessarily a problem).
Er hat zu schnell gegessen. – He ate too fast (and that caused a problem, like feeling sick).

So zu with an adjective/adverb = too (excessively),
while sehr = very (a lot, but not automatically “too much”).

Why is it gegessen hat at the end, and not hat gegessen like in many other sentences?

This is about main clause vs. subordinate clause word order.

In a main clause, the conjugated verb is in second position:
- Er hat zu schnell gegessen.
  – subject Er, verb hat in position 2, participle gegessen at the end.
In a subordinate clause introduced by weil, dass, wenn, etc., the conjugated verb goes to the very end of the clause. If there is also a past participle, you get a verb cluster at the end:
- …, weil er zu schnell gegessen hat.
  – weil introduces the clause, then subject er, adverb zu schnell, and finally the verb cluster gegessen hat at the very end.

So:

Main clause: Er hat zu schnell gegessen.
Subordinate clause: …, weil er zu schnell gegessen hat.

That is the regular rule: in weil-clauses, the finite verb comes last.

Why do we use hat gegessen (perfect tense) instead of the simple past aß?

Both are grammatically possible:

…, weil er zu schnell gegessen hat. – perfect (hat + gegessen)
…, weil er zu schnell aß. – simple past (preterite)

In modern spoken German, especially in the south and in everyday conversation, people strongly prefer the perfect tense for most verbs:

Er hat gegessen. (spoken standard)
Er aß. (more written, literary, or higher style)

The simple past is still very common for a few verbs like sein, haben, werden, können, etc., but for essen, trinken, gehen, machen, and so on, the perfect is more natural in speech.

So the sentence uses hat gegessen because it sounds normal and conversational.

Why is the auxiliary hat used, and not ist (like ist gegangen)?

In the perfect tense, German generally uses:

sein for:
- verbs of motion or change of state (gehen, kommen, sterben, werden, einschlafen, etc.)
- a few special intransitive verbs.
haben for:
- transitive verbs (taking a direct object)
- most other verbs, including essen.

essen is fundamentally a transitive verb:
Er isst den Apfel. – He eats the apple.

So its perfect tense is formed with haben:

Er hat gegessen. – He has eaten / he ate.
…, weil er zu schnell gegessen hat.

Using ist gegessen would be wrong here; that form appears only in passive-like constructions with an object, such as Der Kuchen ist gegessen (literally: The cake is eaten = The cake is gone/finished), which is a different structure.

Why does weil force the verb to the end? Could I say …, weil er hat zu schnell gegessen?

No, …, weil er hat zu schnell gegessen is incorrect in standard German.

Weil introduces a subordinate clause (Nebensatz). In standard grammar, that means:

The conjugated verb (here: hat) goes to the end of the clause.
Any past participle (here: gegessen) appears before the finite verb, so you get gegessen hat as a cluster at the end.

Correct:

…, weil er zu schnell gegessen hat.

Incorrect in standard German:

…, weil er hat zu schnell gegessen.

You may hear verb-second after weil in some dialects or casual speech, but it is not accepted as correct in standard written German.

What is the function of the comma before weil?

The comma marks the beginning of a subordinate clause introduced by weil:

Main clause: Dem Mann ist übel,
Subordinate clause: weil er zu schnell gegessen hat.

In standard German punctuation, you must put a comma before most introduced subordinate clauses:

…, weil …
…, dass …
…, wenn …
…, obwohl …

So the comma is obligatory here.

Does er in weil er zu schnell gegessen hat refer to dem Mann? Could I repeat the noun instead of the pronoun?

Yes, er clearly refers back to dem Mann:

Dem Mann ist übel, weil er zu schnell gegessen hat.
– The man feels sick because he ate too fast.

Since Mann is masculine singular, the matching pronoun is er.

You could also repeat the noun:

Dem Mann ist übel, weil der Mann zu schnell gegessen hat.

This is grammatically correct, but sounds stylistically clumsy and repetitive. Native speakers normally use the pronoun er (or, if the man was mentioned earlier in a different case, the appropriate pronoun in that case).

So the natural pattern is:

Noun in first clause → pronoun referring back in the second clause.