Learn Hardcore German: Der Pullover ist so weich, dass mir sofort warm wird. - The sweater is so soft that I immediately feel warm.

Here, mir is in the dative case and marks the experiencer (the person who feels the warmth). There is no personal subject like ich doing an action.

You can say Ich werde warm, but it sounds more physical and a bit unusual, as if your body were literally heating up. For everyday “I feel (getting) warm”, mir (wird) warm is the natural idiom.

Why is it mir (dative) and not mich (accusative) in dass mir sofort warm wird?

In expressions about physical or emotional states, German often uses the dative of the experiencer:

Mir ist kalt / Mir ist warm.
Mir ist schlecht.
Mir wird schwindlig.

The idea is “to me it is warm”, so mir is dative.

Using mich (accusative) would be wrong here:
✗ dass mich sofort warm wird – ungrammatical in standard German.

Is something missing in dass mir sofort warm wird? Should it be dass es mir sofort warm wird?

The “dummy” subject es is often optional in this type of clause.

Full form: …dass es mir sofort warm wird.
Shorter, completely normal form: …dass mir sofort warm wird.

Both are correct. Native speakers often drop the “es” in spoken and informal written German when it’s clear from context and the verb is a form of sein / werden in such impersonal expressions.

What is the difference between mir wird warm and mir ist warm?

Mir wird warm = I’m (starting to) get warm.
Focus on the change of state: you were colder before, now you are becoming warm.
Mir ist warm = I am warm / I feel warm.
Focus on the current state, not on the process of becoming warm.

In this sentence:

Der Pullover ist so weich, dass mir sofort warm wird.

the pullover causes a change: as soon as you put it on, you become warm. That’s why wird is more natural than ist here.

Could I say Der Pullover ist so weich, dass ich sofort warm werde instead?

Grammatically, yes: …dass ich sofort warm werde is correct.

However, it is less idiomatic for “I (start to) feel warm because of it”. Native speakers overwhelmingly prefer the impersonal dative construction:

…dass mir sofort warm wird.

Ich werde warm can sound more literal or physical (“my body is heating up”), or it may be used in special contexts (e.g. sport, warm-up), but for clothing making you feel warm, mir wird warm is the standard expression.

What does the so … dass construction mean in so weich, dass mir sofort warm wird?

so … dass introduces a result / consequence:

so weich, dass … = so soft that …
First part: degree/intensity (so weich)
Second part: resulting effect (dass mir sofort warm wird)

Pattern:

Er ist so müde, dass er kaum sprechen kann.
Es war so laut, dass ich nichts gehört habe.

So here: The pullover is so soft *that I immediately get warm (because of it).*

Why is the verb wird at the very end of dass mir sofort warm wird?

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

Main clause: Der Pullover ist so weich, …
- Finite verb ist in position 2.
Subordinate clause: … dass mir sofort warm wird.
- Subordinating conjunction: dass
- Verb wird goes to the final position.

This is the standard word order rule:
dass + [other elements] + finite verb (at the end).

Is the comma before dass obligatory?

Yes. In modern standard German, a comma before a “dass”-clause is mandatory, because it marks the beginning of a subordinate clause.

So:

✓ Der Pullover ist so weich, dass mir sofort warm wird.
✗ Der Pullover ist so weich dass mir sofort warm wird. (incorrect in standard writing)

Why so weich and not simply sehr weich?

Both are possible but they express slightly different ideas:

sehr weich = very soft
→ Just describes the degree of softness.
so weich, dass … = so soft that …
→ Describes degree plus a result: so soft that something happens as a consequence.

In this sentence the speaker doesn’t just say the pullover is very soft; they want to highlight the effect of that softness (you immediately get warm). Hence so weich, dass … is more expressive and natural.

Why is Pullover capitalized and why der Pullover?

In German:

All nouns are capitalized, so Pullover needs a capital P.
Pullover (sweater/jumper) is grammatically masculine, so its definite article is der in the nominative singular:
- der Pullover – ein Pullover
- den Pullover (accusative)
- dem Pullover (dative)
- des Pullovers (genitive)

In this sentence, Der Pullover is the subject (nominative), so der is used.