Jeg foretrækker det mærke, fordi tøjet er ret godt.

In this sentence, det mærke most naturally means that brand.

A useful comparison is:

• et mærke = a brand
• mærket = the brand
• det mærke = that brand

So Danish can show definiteness in two different ways:

• with a suffix on the noun: mærket
• with a determiner before the noun: det mærke

If you want to be extra explicit about this or that, Danish often uses:

• det her mærke = this brand
• det der mærke = that brand

Because mærke is a neuter noun.

In Danish, singular nouns are mainly either:

• common gender: take en
• neuter: take et

Since it is et mærke, the matching demonstrative/article form is det:

• en bil → den bil
• et mærke → det mærke

So det mærke is correct because mærke is neuter.

Because tøjet is the definite form: the clothes / the clothing.

Danish usually makes a noun definite by adding the article to the end:

• tøj = clothes / clothing
• tøjet = the clothes / the clothing

Here it sounds natural because the sentence is talking about the clothing from that brand in a specific sense.

Because Danish tøj is a mass noun, not a plural count noun.

So even though English often uses the plural word clothes, Danish treats tøj more like English clothing:

• Tøjet er godt = The clothing is good
• not something like a plural clothes are

That is why the verb is singular (er) and the adjective agrees as singular/neuter.

Because tøj is grammatically neuter singular, and adjectives must agree with it.

Compare:

• en jakke er god = a jacket is good
• et mærke er godt = a brand is good
• tøjet er godt = the clothing is good

So after er, the adjective still agrees with the noun. Since tøj is neuter, you use godt.

Ret here means quite, rather, or pretty.

So ret godt means something like:

• quite good
• rather good
• pretty good

It is a very common everyday word and is usually a bit softer than meget godt (very good).

Because fordi introduces a subordinate clause.

In standard Danish, subordinate clauses do not use main-clause word order rules. So after fordi, the normal order is:

subject + verb + other elements

That gives:

• fordi tøjet er ret godt

A learner may expect something different because Danish main clauses often follow verb-second patterns, but after fordi, standard written Danish keeps the subordinate structure.

Foretrækker is the present tense of at foretrække.

So:

• at foretrække = to prefer
• jeg foretrækker = I prefer

A useful thing to remember is that Danish present tense is the same for all persons:

• jeg foretrækker
• du foretrækker
• han/hun foretrækker
• vi foretrækker

So unlike English, the verb form does not change depending on the subject.

Because fordi introduces a subordinate clause, and many Danish writers place a comma before such clauses.

So this is very normal:

• Jeg foretrækker det mærke, fordi tøjet er ret godt.

You may also see the comma omitted in modern Danish writing, depending on the comma system being used. So both versions can appear, but the comma here is completely natural.

These are two vowels English speakers often find difficult:

• æ in mærke is roughly an open e/a sound, somewhere around the vowel area of English bed or cat, depending on accent.
• ø in tøj has no exact English equivalent. A useful trick is to say e as in bed while rounding your lips.

A couple of practical tips:

• mærke: pay attention to the æ
• tøj: the øj sound is especially important and very Danish-sounding

The best approach is to listen and imitate, because Danish vowel pronunciation is hard to learn accurately from spelling alone.