Sporten er populær.

Because sporten is the definite singular form of en sport.

• en sport = a sport
• sporten = the sport

In Norwegian, the definite article is usually added to the end of the noun:

• en bil → bilen
• en skole → skolen
• en sport → sporten

So Sporten er populær means that a specific sport, or the sport being talked about, is popular.

Er means is / am / are. It is the present tense of å være (to be).

Examples:

• Jeg er trøtt. = I am tired.
• Hun er lærer. = She is a teacher.
• Sporten er populær. = The sport is popular.

Unlike English, Norwegian does not change the present tense form depending on the subject. So er works with I, you, he, she, it, we, and they.

Because it is a predicate adjective: it comes after er and describes the subject.

In Norwegian, predicate adjectives do show some agreement, but not in the same way as adjectives placed before nouns.

Compare:

• En sport er populær.
• Et spill er populært.
• Sporter er populære.

So:

• populær is used with many en-words
• populært is used with et-words
• populære is used in the plural

Since sporten is singular and comes from en sport, populær is correct.

This is a very common question.

When an adjective comes before a noun, definite nouns usually require the -e form:

• den populære sporten = the popular sport

But in Sporten er populær, the adjective comes after the verb er, so it is a predicate adjective, not an attributive adjective. In that position, Norwegian normally uses agreement for gender and number, not a special definite form.

So:

• den populære sporten = adjective before the noun
• sporten er populær = adjective after er

That is why populær is correct here, not populære.

Yes, but it means something slightly different.

• Sporten er populær. = The sport is popular
  This usually refers to a specific sport already known from the context.

• Sport er populært. = Sport is popular
  This talks about sport in a general sense.

So the difference is mainly:

• sporten = a specific sport / the sport
• sport = sport in general

Also notice the adjective:

• Sporten er populær.
• Sport er populært.

When sport is used more like an abstract idea or category, Norwegian often uses populært.

Yes, in standard Bokmål it is normally a common-gender noun and takes en:

• en sport
• sporten

For an English speaker, it is often easiest to think of this as a masculine-type pattern, since the definite ending is -en.

No, this is the normal basic word order for a simple statement:

• Sporten = subject
• er = verb
• populær = adjective/complement

So the pattern is:

subject + verb + complement

That gives: Sporten er populær.

Norwegian main clauses usually follow the V2 rule (the finite verb is in second position). In this sentence, the subject comes first, so the verb naturally comes second.

For example:

• Sporten er populær.
• I Norge er sporten populær.

In both sentences, the verb er is still in second position.

Usually yes. Populær is very close to English popular and can describe something that many people like, enjoy, or are interested in.

Examples:

• Han er populær. = He is popular.
• Denne serien er populær. = This series is popular.
• Sporten er populær. = The sport is popular.

So this is a very useful adjective to learn, and in most everyday cases it works much like the English word.

A rough guide for an English speaker is:

• Sporten ≈ SPOR-ten
• er ≈ a short air / ehr sound, depending on accent
• populær ≈ po-pu-LAIR

A few helpful notes:

• The r sounds are Norwegian r sounds, which vary by dialect.
• The stress in populær is on the last syllable: popuLÆR
• Norwegian pronunciation differs a lot by region, so exact pronunciation may vary

A simple learner-friendly approximation is:

SPOR-ten ehr po-poo-LAIR

That is not perfect Norwegian pronunciation, but it is a reasonable starting point.