Strategien er enkel.

Because Danish usually puts the definite article at the end of the noun as a suffix.

• strategi = strategy
• strategien = the strategy

For many en-words (common gender nouns), the definite singular ending is -en.

So:

• en strategi = a strategy
• strategien = the strategy

This is one of the biggest differences from English, where the comes before the noun.

Er is the present tense of at være (to be).

So:

• jeg er = I am
• du er = you are
• han/hun er = he/she is
• strategien er = the strategy is

In this sentence, er links the subject strategien with the description enkel.

Because the adjective has to match the noun, and strategi is an en-word (common gender singular).

When an adjective is used after er (predicatively), the usual pattern is:

• common gender singular → base form
• neuter singular → often -t
• plural → often -e

So:

• Strategien er enkel. = The strategy is simple.
• Problemet er enkelt. = The problem is simple.
  (problem is a neuter noun, so the adjective becomes enkelt)
• Strategierne er enkle. = The strategies are simple.

You usually have to learn the gender together with the noun:

• en strategi

That tells you it is a common gender noun, not a neuter noun.

In Danish, gender affects:

• the article: en or et
• adjective agreement
• the definite form

So if you learn strategi, it is best to learn it as en strategi.

It often does not need one, because the noun itself carries definiteness.

Compare:

• en strategi = a strategy
• strategien = the strategy

This suffix-style definite article is very common in Danish.

However, Danish can also use a separate definite word in some situations, especially with adjectives before nouns:

• den enkle strategi = the simple strategy

But in your sentence, the adjective comes after er, so you just get:

• Strategien er enkel.

No, this is the normal word order for a simple Danish main clause:

• Subject + verb + complement

So:

• Strategien = subject
• er = verb
• enkel = complement/adjective

This is very similar to English:

• The strategy is simple.

Yes, but it means something different.

• Strategien er enkel. = The strategy is simple.
  (a specific strategy)
• En strategi er enkel. = A strategy is simple.
  (less specific, more general or introducing one strategy)

So the difference is mainly definite vs indefinite.

It is a descriptive adjective meaning simple.

Here it is used after the verb er, so it is functioning as a predicate adjective.

Compare:

• Den enkle strategi = the simple strategy
  (adjective directly before the noun)
• Strategien er enkel = the strategy is simple
  (adjective after er)

Both are correct, but the grammar around definiteness is different.

Yes. A useful basic pattern is:

• enkel → common gender singular
• enkelt → neuter singular
• enkle → plural and many definite/adjectival contexts

Examples:

• en enkel strategi = a simple strategy
• et enkelt problem = a simple problem
• simple / the simple strategies = enkle strategier / de enkle strategier

So enkel is just one form of the adjective.

Yes, sometimes enkel can also mean single in certain contexts, depending on usage.

But in Strategien er enkel, the natural meaning is simple.

Learners should also know that Danish sometimes uses simpel too, but simpel can carry a different tone depending on context. Enkel often sounds like straightforward, uncomplicated, simple.

A rough learner-friendly pronunciation is:

• Strategien ≈ stra-te-GEE-en
• er ≈ a short, weak air/uh sound
• enkel ≈ ENG-kel or EN-kel

A more natural Danish pronunciation will be softer and less fully pronounced than the English-style approximation. The important thing for a beginner is to notice that:

• strategien has the definite ending -en
• er is often unstressed
• enkel has two syllables

If you want to sound more natural, listen to native audio, because Danish pronunciation is often much less phonetic than the spelling suggests.

Yes. It is a full, natural sentence.

It has:

• a subject: Strategien
• a verb: er
• a description: enkel

So even though it is short, it is fully grammatical and complete.