Learn Hardcore German: Äußerlich lache ich, aber innerlich bin ich nervös. - Outwardly I laugh, but inwardly I am nervous.

Because Äußerlich is in first position, the subject ich has to move after the verb, giving Äußerlich lache ich, not Äußerlich ich lache (which would be wrong in standard German).

The same rule explains patterns like:

Heute gehe ich ins Kino. – Today I am going to the cinema.
Morgen fahre ich nach Berlin. – Tomorrow I’m going to Berlin.

Could I also say Ich lache äußerlich, aber ich bin innerlich nervös? Is there a difference?

Yes, Ich lache äußerlich, aber ich bin innerlich nervös is completely correct and natural.

The difference is mainly emphasis:

Äußerlich lache ich, aber innerlich bin ich nervös.
Emphasises the contrast between outside and inside by putting Äußerlich and innerlich in first position.
Ich lache äußerlich, aber ich bin innerlich nervös.
Is a more neutral word order; the focus is slightly more on ich (I).

All of these are grammatically fine, with slightly different emphasis:

Ich lache äußerlich, aber innerlich bin ich nervös.
Äußerlich lache ich, aber ich bin innerlich nervös.

Can I leave out the second ich and say Ich lache äußerlich, aber bin innerlich nervös?

Yes, that is possible:

Ich lache äußerlich, aber bin innerlich nervös.

German often allows you to omit a repeated subject in the second clause if it is clearly the same as in the first clause, especially in spoken or informal language.

However:

For learners, it is often clearer and safer to repeat the subject:
Ich lache äußerlich, aber ich bin innerlich nervös.
You cannot drop the subject in both clauses; at least one ich must be there.

Your original version with ich in both clauses is always correct and never sounds wrong.

Why is there a comma before aber? Could I write it without the comma?

Here, the comma is mandatory.

The sentence contains two full main clauses:

Äußerlich lache ich – subject ich, verb lache
innerlich bin ich nervös – subject ich, verb bin

When aber connects two independent clauses like this, German spelling rules require a comma:

Äußerlich lache ich, aber innerlich bin ich nervös.

You can only leave out the comma when aber connects smaller parts of a sentence, not full clauses, for example:

Es ist teuer(,) aber gut. – It is expensive but good.
Here teuer and gut are just two parts of the predicate; there is only one verb (ist).

In your sentence, both sides have their own verb, so the comma cannot be omitted.

What exactly do äußerlich and innerlich mean here, and are they adjectives or adverbs?

Meaning:

äußerlich – “on the outside, externally, outwardly (in appearance)”
→ how something looks or behaves from the outside
innerlich – “inside, inwardly, emotionally/mentally”
→ what you feel or are like inside

In this sentence they function adverbially:

Äußerlich lache ich – I laugh outwardly (that’s how I appear)
innerlich bin ich nervös – inwardly I am nervous (that’s how I feel inside)

Grammatically, äußerlich and innerlich are adjectives that are being used as adverbs.
Because they are not in front of a noun, they have no endings (no gender/case endings like -e, -en, etc.).

Can äußerlich and innerlich also be used directly before nouns?

Yes. As adjectives before nouns, they take endings and often appear as äußere / inner(e) or äußerliche / innerliche, depending on style and nuance.

Examples:

die äußere Form – the external form
die inneren Werte – the inner values
die äußeren Umstände – the external circumstances
mein inneres Gleichgewicht – my inner balance

Forms with -lich are usually a bit more descriptive/characterising:

die äußerliche Erscheinung – the outward appearance (emphasis on appearance)
die innerliche Unruhe – internal restlessness

In your sentence, though, they act as simple adverbs: äußerlich / innerlich without endings.

Is aber exactly the same as English but? Are there other uses of aber?

In your sentence, aber is a coordinating conjunction that works just like English but, introducing a contrast:

Äußerlich lache ich, aber innerlich bin ich nervös.
→ Outwardly I laugh, but inwardly I am nervous.

However, aber has another common use as a modal particle, where it does not mean “but”:

Das ist aber schön! – That’s really nice! / My, that’s nice!
Du bist aber groß geworden! – You’ve really grown!

In these exclamations, aber adds emphasis or surprise, not contrast.

So:

In your sentence: aber = but / however.
In many exclamations: aber = an emphasiser indicating surprise or strong reaction.

Why is it innerlich bin ich nervös and not innerlich ich bin nervös?

Again, this is the V2 rule in action.

In the second clause, innerlich is placed first for emphasis:

Position 1: innerlich
Position 2: bin (finite verb)
Rest: ich nervös

Correct patterns:

Innerlich bin ich nervös.
Ich bin innerlich nervös.

Wrong in standard German:

*Innerlich ich bin nervös. (violates the verb-second rule)
*Innerlich nervös bin ich. (here the verb is not in second position)

So the verb must be second, no matter what stands first.

How do you pronounce äußerlich, innerlich, and nervös?

Approximate pronunciations:

äußerlich → OY-ser-likh
- äu like oy in English boy
- ß like ss in kiss
- final -lich with the soft German ch as in ich, not like English k
innerlich → IN-ner-likh
- short i as in bit, not like ee
- again, -lich with the soft ch
nervös → roughly ner-VØHS
- e in ner- somewhat like e in bed
- ö somewhere between English e in her and i in bird, but with rounded lips
- final s is voiceless, like s in bus, not like z

In IPA (for reference):

äußerlich – [ˈɔʏsɐlɪç]
innerlich – [ˈɪnɐlɪç]
nervös – [nɛʁˈvøːs]

Is lache the same as lächele? Would lächele fit better?

They are different verbs:

lachen – to laugh
- ich lache – I laugh
lächeln – to smile
- ich lächle or ich lächele – I smile

In your sentence:

Äußerlich lache ich, aber innerlich bin ich nervös.

This suggests you are actually laughing (maybe to hide your nervousness).
If you only want to say you smile outwardly, you could say:

Äußerlich lächle ich, aber innerlich bin ich nervös.
or
Äußerlich lächele ich, aber innerlich bin ich nervös.

Both are grammatically correct; which verb fits better depends on what you want to express (full laugh vs. just a smile).

Could I replace nervös with something like aufgeregt? What is the difference?

Yes, but there is a nuance difference:

nervös – nervous, anxious, jittery; often negative (stage fright, anxiety).
aufgeregt – excited / agitated; can be positive (looking forward to something) or more neutral “worked up”.

So:

innerlich bin ich nervös
→ I am nervous inside (worried, anxious).
innerlich bin ich aufgeregt
→ I am excited / all worked up inside (could be good or bad).

In contexts like an exam or a job interview, nervös is usually the more precise choice.

Is the sentence neutral, formal, informal, or poetic? How natural does it sound?

The sentence is natural and neutral, and it works in:

Spoken language (talking about your feelings)
Informal writing (messages, emails)
More reflective or literary contexts, because of the neat outer/inner contrast

It is not especially formal or old-fashioned, and it is not slang.
The parallel structure Äußerlich … aber innerlich … simply makes it sound a bit stylistically polished, which is why you might see it in books, blogs, or personal essays as well as in conversation.