Learn Hardcore Norwegian: Jeg blir nervøs når sjefen ringer meg.

Jeg blir nervøs = I get/become nervous (a reaction or change that happens when the boss calls).
Jeg er nervøs = I am nervous (a general or current state, without saying what causes it).

So in this sentence, blir is used because the nervousness is triggered by the boss calling.

What exactly does når do here, and how is it different from da or hvis?

Når is a time conjunction meaning when. In this sentence it introduces a time clause for something that can happen repeatedly or in general.

Når sjefen ringer meg = Whenever / when the boss calls me (any time he calls).
Da is normally used for a single event in the past:
- Jeg ble nervøs da sjefen ringte meg = I got nervous when (on that occasion) the boss called me.
Hvis means if and introduces a condition:
- Jeg blir nervøs hvis sjefen ringer meg = I get nervous if the boss calls me (more hypothetical or conditional).

Here når is correct because it describes what generally happens each time the boss calls.

Why is it sjefen and not en sjef?

Sjefen is the definite form: the boss. En sjef is a boss.

Using sjefen usually implies a specific boss that both speaker and listener know about – typically my boss at work. Norwegian often uses the definite form where English would just say my boss or even just the boss with the meaning being clear from context.

If you said en sjef ringer meg, it would sound like some boss is calling you, not necessarily your own boss.

Should it be sjefen min instead of just sjefen?

Both are possible, but they are not identical in feel.

Sjefen ringer meg usually already suggests my boss in a normal context.
Sjefen min ringer meg explicitly says my boss is calling me.

You add min if you want to make it very clear or contrast it with someone else’s boss, but in many everyday situations, sjefen alone is enough and sounds more natural.

Why is the verb in the når-clause present tense ringer, even though this can refer to repeated events?

Norwegian present tense is used both for actions happening now and for general or repeated actions.

So når sjefen ringer meg can mean:

when he is calling me now (if you are describing the current situation), or
whenever he calls me in general (habitual/repeated action).

You do not need a special tense for habitual actions; the simple present ringer covers that.

Could I say ringer til meg instead of ringer meg? What about ringer meg opp?

The most natural standard way to say call someone (by phone) is:

ringe noen → sjefen ringer meg (the boss calls me).

Other options:

ringe til noen (e.g. sjefen ringer til meg) is also correct, but feels a bit longer and sometimes slightly more formal or emphatic; many speakers just drop til.
ringe noen opp (e.g. sjefen ringer meg opp) literally call someone up, often used for call back or make the call, but in modern speech people often just say ringe noen.

In your sentence, sjefen ringer meg is the most natural form.

Why is the word order når sjefen ringer meg and not når ringer sjefen meg?

Inside a subordinate clause introduced by når, Norwegian uses normal Subject–Verb–Object word order:

når sjefen ringer meg → subject (sjefen), then verb (ringer), then object (meg).

Når ringer sjefen meg? is a different structure: that is a main-clause question meaning When is the boss calling me?

So:

Time clause (subordinate): når sjefen ringer meg
Question to ask about time: Når ringer sjefen meg?

Can I put the når-clause at the start of the sentence, and what happens to word order then?

Yes, you can move the når-clause to the front:

Når sjefen ringer meg, blir jeg nervøs.

In Norwegian main clauses, the finite verb has to be in second position, so when the whole når-clause comes first, the verb blir must come before jeg:

Correct: Når sjefen ringer meg, blir jeg nervøs.
Incorrect: Når sjefen ringer meg, jeg blir nervøs.

Inside the når-clause itself, the word order stays subject–verb–object: sjefen ringer meg.

Why is it meg and not jeg at the end?

Jeg is the subject form (I), and meg is the object form (me).

In this sentence:

Jeg is the subject of blir nervøs.
Sjefen is the subject of ringer.
Meg is the object of ringer (the one who is being called).

So you must use meg after ringer, just like in English you say calls me, not calls I.

Why is the adjective nervøs and not nervøse?

Adjectives in Norwegian change form depending on gender/number and whether the noun is definite or plural. In predicative position (after er, blir etc.):

Singular: base form nervøs
- Jeg er nervøs.
- Jeg blir nervøs.
Plural: nervøse
- Vi er nervøse.
- De blir nervøse.

In attributive position, before a noun:

Indefinite singular: en nervøs sjef
Definite singular: den nervøse sjefen

Since the subject is jeg (singular), you use the base form nervøs.

Does blir always mean becomes? Are there other common uses I should know?

Bli often means become, but it has several important uses:

Become / get:
- Jeg blir nervøs. = I get nervous.
Future-like meaning (will be / will become):
- Det blir bra. = It will be fine.
Passive constructions:
- Døren blir åpnet. = The door is opened / is being opened.
Stay / remain in some expressions:
- Bli hjemme. = Stay at home.

In your sentence, it is the straightforward become/get meaning.

Does nervøs mean exactly the same as English nervous?

Nervøs is close to English nervous, but usage overlaps both nervous and anxious / on edge. Typical uses:

Jeg blir nervøs før eksamen. = I get nervous before exams.
Han virker litt nervøs. = He seems a bit tense/nervous.

If you mean more afraid/scared, Norwegian often uses redd:

Jeg blir redd når sjefen roper. = I get scared when the boss shouts.

For stressed, you more often hear stresset or stressa:

Jeg blir stressa når sjefen ringer meg.

How do you pronounce sjefen and når? The spelling looks confusing.

Some pronunciation tips (standard Eastern Norwegian):

sjefen:
- sj is like English sh in she.
- e is like e in bed.
- Final -en is roughly en.
- Approximate: SHEH-fen.
når:
- The vowel is similar to the long vowel in British law, with rounded lips.
- The r is usually a tapped or rolled r in many accents.
- Approximate: nohr (one syllable).

So the whole sentence roughly: Yeg blir nerveus når SHEH-fen ringer meg. (very approximate for an English ear).