Learn Hardcore Dutch: Sommige fans zijn jaloers als hun team geen kampioen wordt. - Some fans are jealous when their team doesn’t become champion.

sommige fans = some fans (others are not like that)
een paar fans = a few fans (small number; more about quantity than contrast)
enkele fans = a few / some fans (a bit more formal or written than een paar)

In this sentence, sommige fans nicely highlights that only part of all fans become jealous, not everyone.

Why is it sommige fans zijn and not sommige fans is?

Because fans is plural, the verb must also be plural:

de fan is (singular)
de fans zijn (plural)

So:

Sommige fan is jaloers… → singular (unnatural here)
Sommige fans zijn jaloers… → correct plural form

How is jaloers used in Dutch? Do you need a preposition like "jealous of"?

On its own, jaloers is an adjective meaning jealous / envious.

With an object, you almost always say jaloers op:

Ik ben jaloers op hem. – I am jealous of him.
Ze is jaloers op haar vriendin. – She is jealous of her friend.

In your sentence there is no explicit object, so jaloers stands alone:

Sommige fans zijn jaloers…
→ Implies: jealous of other teams, of the winner, of fans whose team became champion, etc.

Why is als used here instead of wanneer or toen?

Als introduces a condition or a general situation, often translated as if or when (whenever):

Sommige fans zijn jaloers als hun team geen kampioen wordt.
→ Whenever / if their team doesn’t become champion.

Wanneer is possible here too and is a bit more formal or neutral about time:

… wanneer hun team geen kampioen wordt. – stylistically fine, more formal/neutral.

Toen is only used for a specific event in the past, not for general conditions:

Toen hun team geen kampioen werd, waren ze jaloers.
→ Then/when their team didn’t become champion (on that specific occasion).

So als is correct because we’re talking about a general, repeatable situation.

Why does the verb wordt go to the end of the clause after als?

In Dutch, main clauses usually have the finite verb in second position (V2 word order):

Hun team wordt geen kampioen.
(subject hun team, verb wordt, rest geen kampioen)

But in a subordinate clause introduced by a conjunction like als, omdat, dat, wanneer, the finite verb normally goes to the end:

… als hun team geen kampioen wordt.
(als [conjunction] + hun team [subject] + geen kampioen [rest] + wordt [finite verb at the end])

So als triggers subordinate clause word order with the verb final.

What is the difference between wordt kampioen and is kampioen?

kampioen worden = to become champion / to win the championship
→ focuses on the change or the result of a competition.
kampioen zijn = to be champion
→ focuses on the state of being champion.

Your sentence:

… als hun team geen kampioen wordt.
→ when their team does not become champion (doesn’t win the title).

If you said:

… als hun team geen kampioen is.
→ when their team is not champion (not in the champion state), which feels more like a static situation, not specifically about winning/losing a season.

Why is it geen kampioen instead of niet kampioen?

Geen is used to negate an indefinite noun (often where English has no or not a):

een kampioen → geen kampioen (a champion → no champion / not a champion)
een boek → geen boek (a book → no book)

Niet is used to negate:

verbs, adjectives, adverbs, prepositional phrases,
definite nouns (with de, het, mijn, dit, etc.),
or a predicate noun after zijn.

So:

Hun team wordt geen kampioen.
→ Their team does not become champion / becomes no champion.

Compare:

Hun team is niet kampioen.
→ Their team is not champion. (predicate after is, so niet)

What exactly is hun in hun team, and how is it different from zij or hen?

Here, hun is a possessive pronoun meaning their:

hun team = their team
hun huis = their house

Basic pronoun set (standard grammar):

zij / ze = they (subject)
- Zij winnen. – They win.
hen = them (direct object, after prepositions)
- Ik zie hen. – I see them.
- Ik ga met hen. – I go with them.
hun = their (possessive, before a noun)
- hun team – their team

So in your sentence, hun simply marks possession: the team that belongs to them.

Why is the plural of fan written as fans and not fanen?

Fan is a loanword from English, and many modern loanwords in Dutch form the plural with -s:

de fan → de fans
de club → de clubs
de hobby → de hobby’s
de film → de films

The ending -en is more common with older, native words:

de hond → de honden
de boom → de bomen

So fans is the normal, correct plural form in Dutch.

Could the sentence also be Sommige fans zijn jaloers wanneer hun team geen kampioen wordt? Is there any difference?

Yes, that sentence is also correct:

Sommige fans zijn jaloers als hun team geen kampioen wordt.
Sommige fans zijn jaloers wanneer hun team geen kampioen wordt.

Both can mean Some fans are jealous when/whenever their team doesn’t become champion.

Subtle nuance:

als is very common in spoken Dutch and often has a conditional feel (if / whenever).
wanneer can sound a bit more formal or neutral, and is slightly more focused on time (when rather than if).

In everyday speech, als is more frequent here, but wanneer is not wrong.

Does jaloers always mean "jealous", or can it also mean "envious"?

Dutch jaloers covers both English jealous and envious. Context decides which English word fits best:

jaloers op iemands succes
→ envious of someone’s success
jaloers op iemands nieuwe vriendin
→ jealous/envious of someone’s new girlfriend
jaloerse partner
→ a jealous partner (possessive jealousy)

So yes, jaloers can express both jealousy and envy; Dutch doesn’t sharply separate the two concepts lexically.