Learn Hardcore Norwegian: Når jeg ser en gråtende tenåring, blir jeg trist. - When I see a crying teenager, I become sad.

Når is used for something that happens in general / repeatedly (a habitual situation): Whenever I see… I become…
Da is typically used for a specific time in the past (a one-off situation): When I saw… I became…
So Når jeg ser… sounds like a general rule about your feelings, not one particular episode.

Why is there a comma after tenåring?

Norwegian normally uses a comma to separate a subordinate clause from the main clause.
Here, Når jeg ser en gråtende tenåring is the subordinate clause, and blir jeg trist is the main clause—so the comma is expected.

Why is the verb before the subject in blir jeg trist? Why not jeg blir trist?

This is Norwegian V2 word order in main clauses: the finite verb comes in the second position.
Because the sentence begins with the subordinate clause (Når …), the main clause starts after the comma, and the verb takes second position there:

Når …, blir jeg trist. (verb blir comes before jeg)
If you start directly with the subject, you get:
Jeg blir trist når jeg ser en gråtende tenåring.

Why is ser (present tense) used here, not past tense?

Because Når plus present tense commonly expresses a general/habitual meaning in Norwegian: Whenever I see…
If you meant a single past event, you’d more likely use:

Da jeg så en gråtende tenåring, ble jeg trist. (past: så, ble)

What’s the difference between blir jeg trist and er jeg trist?

blir trist = become / get sad (a change of state)
er trist = am sad (a state, not focusing on the change)
So blir jeg trist emphasizes that seeing the teenager causes you to become sad.

What part of speech is gråtende, and why doesn’t it change form here?

gråtende is a present participle used like an adjective: crying.
Participles ending in -ende often stay the same across genders and numbers in this kind of use:

en gråtende tenåring (a crying teenager)
You’ll also see regular adjectives that do change: en trist gutt / et trist barn / triste barn.

Why is it en gråtende tenåring—what does en tell me?

en is the indefinite article for common gender nouns (masculine or feminine in many dialects).
It tells you tenåring is a common-gender noun:

en tenåring = a teenager
(If it were neuter, it would be et.)

Could I say ei tenåring instead of en tenåring?

Sometimes, depending on dialect and style. Norwegian has:

en (masc. / common)
ei (fem.)
et (neuter)

Many speakers use en for most common-gender nouns in Bokmål, even if a feminine form exists. Tenåring is often treated as common gender with en; ei tenåring is possible for speakers who actively use feminine forms, but en tenåring is the safest standard choice.

Is tenåring gender-specific, like “teenage boy/girl”?

No—tenåring is gender-neutral: a teenager.
If you want to specify, you can add:

en tenåringsgutt (a teenage boy)
en tenåringsjente (a teenage girl)

Why is trist not triste here?

Because trist agrees with jeg (a single person) and functions as a predicative adjective after the verb (bli). In this use, it doesn’t take plural form:

Jeg blir trist. (singular)
You’d use triste for plural subjects:
Vi blir triste. (We become sad.)

Can I move the clauses around without changing the meaning?

Yes. Both are natural:

Når jeg ser en gråtende tenåring, blir jeg trist.
Jeg blir trist når jeg ser en gråtende tenåring.

The meaning stays the same; the first version puts more focus on the condition/time clause by placing it first.

How would pronunciation likely sound for key words here?

A rough guide (varies by dialect):

Når: like nor (often with a more open vowel)
ser: like sehr (short “eh”)
gråtende: roughly GRAW-tuh-nuh (stress on grå-)
tenåring: roughly TEH-noh-ring (stress on te-)
blir: roughly bleer
trist: like trist with a clear t at the end