Learn Hardcore German: Wenn ich mit meiner Cousine in Kanada telefoniere, muss ich wegen der Zeitverschiebung lange wach bleiben. - When I call my cousin in Canada, I have to stay awake for a long time because of the time difference.

Then the main clause follows: muss ich … lange wach bleiben, where the verb is in second position as usual in main clauses. So the comma marks the boundary between the subordinate clause and the main clause.

Why is it wenn and not wann or als?

All three words can relate to time, but they are used differently:

wenn = when / whenever / if (for repeated actions or general conditions, and also for future events)
- Wenn ich mit ihr telefoniere, bleibe ich lange wach. – Whenever I talk to her on the phone, I stay up late.
wann = when? used only in questions (direct or indirect)
- Wann telefonierst du mit ihr? – When do you call her?
- Ich weiß nicht, wann ich mit ihr telefoniere.
als = when for single events in the past
- Als ich in Kanada war, habe ich sie besucht. – When I was in Canada, I visited her (one specific past time).

In the sentence, the speaker talks about a situation that can happen repeatedly (or generally), so wenn is correct.

Why is it mit meiner Cousine and not mit meine Cousine? What case is meiner?

The preposition mit always takes the dative case.

Cousine is feminine: die Cousine.
Feminine in the dative singular is der, and the corresponding form of meine in dative feminine is meiner.

So:

Nominative: meine Cousine (my cousin – subject)
Dative: mit meiner Cousine (with my cousin)

That’s why you need meiner after mit.

Why is it Cousine and not a word like Vetter? Is Cousine always female?

Yes:

die Cousine = female cousin
der Cousin = male cousin

The more traditional German words are:

die Kusine / die Base (regional/old-fashioned for female cousin)
der Vetter (male cousin, somewhat old-fashioned or regional)

In modern standard German, Cousine (female) and Cousin (male) are very common and neutral. Here, meiner Cousine clearly refers to a female cousin.

Why is it in Kanada and not nach Kanada?

In Kanada is used because the sentence describes location, not movement:

in Kanada = in Canada (where she is / where the cousin lives)
nach Kanada = to Canada (movement to the country)

Examples:

Sie lebt in Kanada. – She lives in Canada.
Ich fliege nach Kanada. – I am flying to Canada.

In your sentence, the speaker is calling someone who is in Canada, so in Kanada is correct.

What’s the difference between telefonieren and anrufen? Why use telefoniere here?

Both are about phone calls, but they work differently in a sentence:

telefonieren (mit) = to talk on the phone (with someone)
- Structure: telefonieren + mit + dative
- Ich telefoniere mit meiner Cousine. – I’m on the phone with my cousin.
jemanden anrufen = to call someone (on the phone)
- Structure: jemanden (accusative) + anrufen (separable verb)
- Ich rufe meine Cousine an. – I call my cousin.

In the sentence, the focus is on the activity of being on the phone with her, so telefonieren mit fits well. You could also say:

Wenn ich meine Cousine in Kanada anrufe, … – correct, just a slightly different phrasing.

Why is there a comma before muss ich?

The comma separates:

the subordinate clause: Wenn ich mit meiner Cousine in Kanada telefoniere
from the main clause: muss ich wegen der Zeitverschiebung lange wach bleiben.

In German, subordinate clauses must be separated by a comma from the main clause. So placing a comma before muss is required because the wenn-clause ends there.

Why is the word order muss ich … lange wach bleiben and not something else?

In the main clause:

The finite (conjugated) verb must be in second position: muss.
The subject ich usually follows the verb: muss ich.
Other elements go next: wegen der Zeitverschiebung, lange.
The infinitive of the main verb (bleiben) goes to the end, because müssen is a modal verb.

So the structure is:

muss (modal verb, 2nd position)
ich (subject)
wegen der Zeitverschiebung (reason)
lange (adverb)
wach (predicate adjective)
bleiben (infinitive at the end)

German likes to put the non-conjugated verb at the end when there’s a modal verb.

Why is it wegen der Zeitverschiebung and not wegen die Zeitverschiebung? What case is that?

The preposition wegen traditionally takes the genitive case, but in everyday spoken German it’s very common to use the dative, especially in the singular.

Here:

Feminine noun: die Zeitverschiebung
Genitive singular: der Zeitverschiebung
Dative singular: der Zeitverschiebung (looks the same for feminine)

So wegen der Zeitverschiebung is grammatically:

standard as genitive, and
also matches colloquial dative forms.

You definitely cannot say wegen die Zeitverschiebung, because die is nominative/accusative feminine, and wegen does not take those cases.

What exactly does Zeitverschiebung mean, and how is the word built?

Zeitverschiebung is a compound noun:

die Zeit = time
die Verschiebung = shift, displacement

So die Zeitverschiebung literally means a shift in time, i.e. the time difference / time zone difference between places. That’s why wegen der Zeitverschiebung is naturally translated as because of the time difference (time zone difference).

What does lange do in lange wach bleiben? Is it an adjective or adverb?

In lange wach bleiben, lange is an adverb meaning for a long time.

wach = awake (adjective used as a predicate: to be/stay awake)
lange = for a long time (adverb describing how long)

So the phrase means to stay awake for a long time. Compare:

Ich bleibe lange wach. – I stay awake for a long time.
Ich war gestern lange wach. – I was awake for a long time yesterday.

Could I say lange aufbleiben instead of lange wach bleiben? Is there a difference?

Yes, you can say lange aufbleiben, and it is very natural:

aufbleiben = to stay up (not go to bed yet)
wach bleiben = to remain awake

Nuance:

lange aufbleiben focuses on not going to bed.
lange wach bleiben focuses on being awake (even if you’re already in bed).

In many everyday contexts, they are almost interchangeable:

Ich muss wegen der Zeitverschiebung lange aufbleiben. – Very idiomatic.
Ich muss wegen der Zeitverschiebung lange wach bleiben. – Also fine, slightly more neutral.

Why is the present tense used (telefonieren, muss … bleiben) even though this can refer to the future? Why not werde bleiben or something similar?

German uses the present tense much more often than English to talk about:

scheduled future events, and
general or repeated situations that include the future.

Context decides whether it’s present or future:

Wenn ich morgen mit meiner Cousine in Kanada telefoniere, muss ich lange wach bleiben.
→ Clearly future because of morgen, but present tense is used.

You can use the future tense:

… werde ich lange wach bleiben müssen.

But that sounds heavier and is usually unnecessary in everyday speech. The present tense is simpler and completely natural for future meanings when the time reference is clear from context.