Learn Hardcore German: Im Bus ist es so warm, dass mir heiß wird und ich anfange zu schwitzen. - On the bus it is so warm that I get hot and start to sweat.

in + dem Bus (preposition + definite article, dative) → im Bus
This contraction is very common and usually preferred in speech and writing.
In dem Bus is grammatically correct but sounds more formal or emphatic in everyday language.

So Im Bus literally means “in the bus”.

Why does the sentence start with Im Bus instead of Es ist so warm im Bus?

German word order is flexible in what can stand at the beginning of a main clause. The main rule is:

The finite verb (here ist) must be in second position.
Anything (subject, object, adverbial) can stand in first position to be emphasized or topicalized.

So we have two acceptable versions:

Im Bus ist es so warm, ... – emphasizes the place (in the bus).
Es ist im Bus so warm, ... or Es ist so warm im Bus, ... – emphasizes the fact that it is warm.

All are correct; the original just foregrounds the location.

What is the function of es in Im Bus ist es so warm?

This es is a dummy (expletive) subject, similar to English “it” in “It is warm in the bus.”

German, like English, normally needs a grammatical subject.
For weather and temperature expressions, German often uses es:
- Es ist warm.
- Es regnet.
- Es ist kalt im Zimmer.

You cannot simply leave es out here; Im Bus ist so warm is incorrect.

How does the so … dass structure work in this sentence?

so … dass expresses a degree plus its consequence, comparable to English “so … that …”:

Im Bus ist es so warm, dass …
→ It is so warm in the bus that …

General pattern:

so
- adjective/adverb + , dass
  - clause
Example: Er ist so müde, dass er einschläft.
→ He is so tired that he falls asleep.

Note that dass introduces a subordinate clause; the conjugated verb goes to the end of that clause.

Why is the verb at the end in dass mir heiß wird?

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

Pattern:

dass
- [other elements] + finite verb (at the end)

In the sentence:

dass mir heiß wird
- mir = dative pronoun
- heiß = predicate adjective
- wird = finite verb → goes to the end

This contrasts with main-clause order:

Mir wird heiß. (main clause: verb in second position)
…, dass mir heiß wird. (subordinate clause: verb at the end)

What case is mir, and why is it mir heiß wird instead of ich werde heiß?

mir is dative case (1st person singular).

German often uses the dative + adjective pattern to describe how someone feels physically:

mir ist heiß / mir wird heiß – I am (getting) hot
mir ist kalt – I am cold
ihm ist schlecht – he feels sick
uns wird langweilig – we are getting bored

Using ich werde heiß would suggest “I become hot” as an object (hot to the touch, or potentially “sexy” in some contexts), which is not what is meant here. The idiomatic way to express personal sensations is:

mir ist/wird heiß, mir ist kalt, mir ist schwindlig, etc.

Is there a difference between mir ist heiß and mir wird heiß?

Yes, there is a nuance:

mir ist heiß → state: I am hot (already).
mir wird heiß → change: I am getting hot, I start to feel hot.

In the sentence:

…, dass mir heiß wird und ich anfange zu schwitzen.

The idea is: because it is so warm in the bus, I start to feel hot, and as a result I start to sweat. The use of wird underlines that this feeling is developing.

Why is it dass mir heiß wird und ich anfange … and not dass mir heiß wird und ich fange an …?

Because both parts (mir heiß wird and ich anfange zu schwitzen) belong to the same subordinate clause introduced by dass.

Structure:

…, dass [mir heiß wird] und [ich anfange zu schwitzen].

In a subordinate clause, the finite verb goes to the end:

… dass mir heiß wird.
… dass ich anfange zu schwitzen.

When you connect them with und under the same dass, you keep subordinate clause word order in both parts. So:

✅ …, dass mir heiß wird und ich anfange zu schwitzen.
❌ …, dass mir heiß wird und ich fange an zu schwitzen.
(This wrongly switches back to main-clause word order in the second part.)

That’s why you see anfange at the end and not fange an.

Where is the separable prefix an in anfange? Shouldn’t it be fange … an?

anfangen is indeed a separable verb.

In a main clause, the prefix separates:
- Ich fange an zu schwitzen. (finite verb 2nd: fange; prefix an at the end)
In a subordinate clause (with dass, weil, etc.), the entire verb, including the prefix, stays together at the end:
- …, dass ich anfange zu schwitzen.

So anfange here is the full verb form placed at the very end of the subordinate clause. That’s why you do not say anfange zu schwitzen an.

Why is anfange in the middle of the clause but zu schwitzen at the very end?

In the subordinate clause part … und ich anfange zu schwitzen:

anfange is the finite verb.
zu schwitzen is an infinitive construction (zu + infinitive).

German rules:

In subordinate clauses, the finite verb goes to the end.
In constructions with zu + infinitive, the zu + infinitive goes after the finite verb.

So the order is:

ich (subject)
anfange (finite verb)
zu schwitzen (zu + infinitive at the very end)

Complete unit: ich anfange zu schwitzen.

Why is there no comma before zu schwitzen?

In modern German spelling, short infinitive groups with zu that are tightly connected to the verb do not require a comma, and often it is omitted:

ich anfange zu schwitzen
er versucht zu schlafen

A comma is required when:

The infinitive group is introduced by um, ohne, statt, anstatt, außer, als:
- …, ohne zu schwitzen.
Or when the infinitive group is more complex or needs clearer separation.

Here, anfange zu schwitzen is a short, closely linked verb–infinitive group, so no comma is necessary and the usual style is to leave it out.

Why is schwitzen not reflexive (like mich schwitzen)?

schwitzen is not a reflexive verb in German. It is an ordinary intransitive verb:

ich schwitze – I sweat
du schwitzt
er/sie schwitzt

You do not say ich schwitze mich. Compare:

sich waschen – reflexive: ich wasche mich (I wash myself)
schwitzen – non‑reflexive: ich schwitze (I sweat)

So anfange zu schwitzen simply means “start to sweat.”

What is the difference between warm and heiß in this sentence?

Both are temperature adjectives, but with different strengths:

warm – warm, not cold; pleasant or moderately high temperature.
heiß – hot; stronger than warm, can be uncomfortably or dangerously hot.

In the sentence:

Im Bus ist es so warm – the bus is (already) very warm.
dass mir heiß wird – it gets hot for me; I personally feel overheated.

So the environment is described as warm, and the personal sensation as hot, which naturally leads to schwitzen (sweating).

Could the sentence also be Es ist so warm im Bus, dass mir heiß wird und ich anfange zu schwitzen?

Yes, that is perfectly correct as well:

Es ist so warm im Bus, dass mir heiß wird und ich anfange zu schwitzen.

The meaning is the same. The difference is only in focus:

Im Bus ist es so warm, … – highlights the location first.
Es ist so warm im Bus, … – starts more neutrally with es, adds location after.

Both satisfy the verb‑second rule in the main clause and are normal German.