Learn Hardcore Dutch: Als ik me nerveus voel, adem ik diep in en denk ik aan de zee. - When I feel nervous, I breathe in deeply and think of the sea.

You need the reflexive pronoun me (myself) with voelen in this sense.
Without me, ik voel nerveus sounds incomplete or wrong to a native speaker.

What is the role of "Als" in "Als ik me nerveus voel", and why is "voel" at the end?

Als introduces a subordinate clause (a dependent clause) meaning "if / when" in this context.

In Dutch subordinate clauses, the finite verb (here: voel) normally goes to the end:

Als
- subject + other elements + verb at the end
  - Als ik me nerveus voel
  - Als hij thuiskomt (When/if he comes home)
  - Als het regent (If/when it rains)

So the word order ik me nerveus voel is typical subordinate-clause order: subject → (object/reflexive) → adjective → verb.

Why is it "adem ik diep in" instead of "ik adem diep in" after the comma?

When a sentence starts with something before the main clause (like a time or condition clause starting with Als), Dutch keeps the verb in second position by inverting the word order.

Base word order (no extra element in front):

Ik adem diep in. (Subject first, then verb)

But when you start with Als ik me nerveus voel, that whole part counts as position 1 of the main sentence. The finite verb of the main clause must then come in position 2, so the subject and verb swap:

Als ik me nerveus voel, adem ik diep in.
(… , adem (verb) ik (subject) …)

This is the standard verb-second (V2) rule in Dutch main clauses.

What’s going on with "adem ik diep in"? Is "inademen" a single verb?

Yes. Inademen is a separable verb:

in (particle) + ademen (to breathe)

In the infinitive or at the end of a clause, it appears as one word:

Ik probeer diep in te ademen. (I try to breathe in deeply.)

But in a main clause in the present tense, the particle separates and usually goes to the end:

Ik adem diep in.
Als ik me nerveus voel, adem ik diep in.

So:

Infinitive: inademen
Conjugated main clause: ik adem … in

Why is it "denk ik aan de zee" and not just "denk ik de zee"?

The verb denken (to think) normally needs a preposition when you say what you’re thinking about. In Dutch that preposition is usually aan:

denken aan
- noun/pronoun
  - Ik denk aan jou. (I think of you.)
  - Ik denk aan mijn werk. (I think about my work.)
  - Ik denk aan de zee. (I think of the sea.)

Without aan, denk ik de zee is ungrammatical in this meaning.

Note:

denken over exists too, but is more “to think about something in order to form an opinion”:
- Ik denk over het probleem na. (I reflect on the problem.)

Why is it "de zee" and not "het zee" in "aan de zee"?

Every Dutch noun has a grammatical gender:

de-words (common gender)
het-words (neuter)

Zee (sea) is a de-word, so you must say:

de zee (the sea)
aan de zee

You just have to learn the gender of each noun. There is no rule that would make zee take het; het zee is always wrong.

Could I also say "Als ik nerveus ben" instead of "Als ik me nerveus voel"? Is there a difference?

Yes, you can say Als ik nerveus ben, and it’s correct. The difference is subtle:

Als ik nerveus ben
Focus: the state: “When I am nervous.”
Als ik me nerveus voel
Focus: the subjective feeling: “When I feel nervous.”

In practice, they often overlap. Ik voel me nerveus sounds a bit more like you’re talking about your internal feelings at that moment, but both are natural.

Why is the reflexive pronoun "me" and not "mij" in "ik me nerveus voel"?

Dutch has weak/unstressed forms and strong/stressed forms of pronouns:

1st person singular:
- unstressed: me
- stressed: mij

Reflexive pronouns in sentences like this are usually unstressed, so you use me:

Ik voel me nerveus.

You would only use mij to add strong emphasis:

Ik voel mij niet schuldig, hoor!
(I don’t feel guilty, me, that’s for sure!)

In your sentence there’s no special emphasis, so me is correct and natural.

Where exactly can "diep" go in "adem ik diep in"? Could I say "ik adem in, diep" or "ik adem diep"?

The neutral, most natural position is:

Ik adem diep in.
Als ik me nerveus voel, adem ik diep in.

Here, diep comes right after the verb and before the separable particle in.

Other options:

Ik adem in, diep.
Sounds unusual and a bit poetic or spoken-with-a-pause.
Ik adem diep.
Grammatically OK, but usually feels incomplete; natives expect in or uit (in or out).

So for standard Dutch, stick with:

adem ik diep in

Why is the subject "ik" repeated in "adem ik diep in en denk ik aan de zee"? Can we drop the second "ik"?

You can optionally drop the second ik here:

Als ik me nerveus voel, adem ik diep in en denk ik aan de zee.
Als ik me nerveus voel, adem ik diep in en denk aan de zee.

Both are correct. Repeating ik is slightly clearer and more formal; leaving it out is more conversational and fluent.

However, in many combinations where the subject is clearly the same, Dutch often leaves the second ik out, especially in speech.

Why is everything in the present tense (voel, adem, denk) even though this sounds like a general habit?

Dutch uses the present tense not only for actions happening right now, but also for:

general truths and habits
things you do regularly or whenever a certain condition is true

So:

Als ik me nerveus voel, adem ik diep in en denk ik aan de zee.
= Whenever I feel nervous, I breathe in deeply and think of the sea.

Using present tense here is exactly parallel to English:
“I feel, I breathe, I think” to describe a regular coping strategy.