Jeg tager en anden, hvis den her er for tung.

In Danish, the present tense is very often used for a future action when the situation is clear from context.

So jeg tager en anden literally looks like I take another one, but in natural English it is often I’ll take another one.

• jeg tager = I take / I’m taking / I’ll take
• jeg vil tage = I want to take / I will take

Be careful: vil often adds an idea of willingness or intention, not just simple future.

En anden means another one.

The noun is left out because it is understood from context. For example, if you are talking about a bag, box, stone, or suitcase, Danish can simply say:

• Jeg tager en anden = I’ll take another one

If you want to include the noun, you could say something like:

• Jeg tager en anden taske = I’ll take another bag

So en anden is functioning like another one in English.

Because anden has to agree with the gender and number of the noun it refers to.

The basic forms are:

• en anden = another (common gender singular)
• et andet = another (neuter singular)
• andre = other / others (plural or sometimes more general use)

Examples:

• en anden stol = another chair
• et andet bord = another table
• andre stole = other chairs

In your sentence, the implied noun is a common-gender singular noun, so en anden is correct.

Den her is a very common spoken and everyday way to say this one or this ... here.

• den is used for a common-gender noun
• her means here, but together den her works like this

So:

• den her = this one / this one here

If the missing noun were included, it could be:

• den her taske = this bag

Since the noun is omitted, den her means this one.

Because den and det depend on grammatical gender.

• den = for en-words (common gender)
• det = for et-words (neuter)

So if the thing being referred to is an en-word, you say:

• den her

If it is an et-word, you say:

• det her

Examples:

• den her stol = this chair
• det her bord = this table

Your sentence uses den her, so the implied noun is a common-gender noun.

Here for means too.

So:

• for tung = too heavy

This is a very common pattern in Danish:

• for stor = too big
• for dyr = too expensive
• for langsom = too slow

English speakers sometimes confuse this with the preposition for, but in this sentence it is an adverb meaning too.

Yes, here hvis means if in the sense of a condition.

• hvis den her er for tung = if this one is too heavy

It introduces a conditional clause.

A useful comparison:

• hvis = if
• fordi = because
• når = when

So in your sentence, hvis sets up the condition for taking another one.

Because after hvis, Danish uses subordinate clause word order.

In this clause:

• den her = subject
• er = verb
• for tung = complement

So the structure is straightforward:

• hvis den her er for tung = if this one is too heavy

This is similar to English. Danish word order becomes more noticeably different when there are adverbs like ikke:

• Hvis den her ikke er god = If this one is not good

In subordinate clauses, words like ikke usually come before the finite verb’s complement area, which learners notice quite quickly.

Yes, but it sounds different in style.

• den her = very common, everyday, natural in speech
• denne = more formal, written, or sometimes more pointed

So:

• hvis den her er for tung = very natural in conversation
• hvis denne er for tung = understandable, but less conversational

Most learners will hear den her much more often in everyday Danish.

Sometimes, yes, but it does not mean exactly the same thing.

• en anden = another one, a different one
• en til = one more, an additional one

So:

• Jeg tager en anden = I’ll take another one instead
• Jeg tager en til = I’ll take one more

In your sentence, the idea is this one won’t do, so I’ll choose a different one, so en anden is the best choice.

Yes. Tung is the basic/common form of the adjective heavy.

After er, Danish usually uses the predicate form of the adjective:

• den er tung = it is heavy
• den er for tung = it is too heavy

Compare that with adjective endings before nouns:

• en tung taske = a heavy bag
• et tungt bord = a heavy table

So in your sentence, tung is correct because it comes after er.