Når jeg læser en svær tekst, plejer jeg både at markere nye ord og understrege de vigtigste sætninger.

Because Danish main clauses normally follow the V2 rule: the finite verb must come in the second position.

Here, the sentence begins with the subordinate clause:

Når jeg læser en svær tekst

After that, the main clause starts, and the finite verb must come first in that clause:

• plejer jeg ...

So the pattern is:

• Når ... , plejer jeg ...

If you started directly with jeg, then you would normally say:

• Jeg plejer både at markere nye ord og understrege de vigtigste sætninger, når jeg læser en svær tekst.

Both are correct, but the word order changes depending on what comes first.

Plejer means something like usually do, tend to do, or be in the habit of doing.

So:

• jeg plejer at markere = I usually mark
• jeg plejer at understrege = I usually underline

It expresses a habit, not something happening just once.

A very common pattern is:

• pleje at + infinitive

For example:

• Jeg plejer at stå tidligt op. = I usually get up early.
• Hun plejer at drikke te. = She usually drinks tea.

Because Når jeg læser en svær tekst is a subordinate clause, and in Danish subordinate clauses are normally separated by a comma.

So:

• Når jeg læser en svær tekst, plejer jeg ...

This is standard Danish punctuation.

English is less strict about this kind of comma in some cases, so this often stands out to English speakers.

Here Når can be understood as when in a habitual sense, which is often close to whenever in English.

So the idea is:

• When/Whenever I read a difficult text, I usually...

Because the main clause contains plejer (usually), the whole sentence describes something repeated or habitual.

Compare:

• Når jeg læser en svær tekst, plejer jeg ...
  = When/Whenever I read a difficult text, I usually...

This is different from a one-time past event, where Danish often uses da instead.

Både ... og means both ... and.

So:

• både at markere nye ord og understrege de vigtigste sætninger
• both to mark new words and underline the most important sentences

It links two things that are both true or both done.

Other examples:

• Jeg kan både synge og danse. = I can both sing and dance.
• Hun drikker både kaffe og te. = She drinks both coffee and tea.

Because the two infinitives are coordinated, and Danish often uses at only once.

So this part means:

• både at markere nye ord og understrege de vigtigste sætninger

This is understood as:

• både at markere ... og at understrege ...

The second at is omitted because it would be repetitive.

You may sometimes see both, but using at only once is very common and natural.

Because tekst is a common gender noun in Danish, so it takes en.

• en tekst = a text

Adjectives must agree with the noun phrase:

• en svær tekst = a difficult text

If the noun were a neuter noun with et, the adjective would usually take -t in the singular indefinite form:

• et svært ord = a difficult word

So:

• en tekst → en svær tekst
• et ord → et svært ord

This means the most important sentences.

There are two things happening here:

1. sætninger is plural of sætning

   • en sætning = a sentence
   • sætninger = sentences

2. vigtigste is the superlative form of vigtig

   • vigtig = important
   • vigtigere = more important
   • vigtigst = most important

When the superlative is used before a noun, Danish normally uses the -e form and often a definite determiner:

• de vigtigste sætninger = the most important sentences

Compare:

• den vigtigste sætning = the most important sentence
• de vigtigste sætninger = the most important sentences

Usually, vigtigste is the normal and most natural form here.

Danish often forms the superlative with -st:

• vigtig → vigtigst / vigtigste

Using mest is possible with some adjectives, especially longer or less easily inflected ones, but with vigtig, vigtigste is the standard choice.

So in this sentence:

• de vigtigste sætninger is the natural form.

They are similar, but not identical.

• markere = to mark, highlight, or indicate
• understrege = to underline

So:

• markere nye ord means you somehow mark the new words, maybe with a highlighter, circle, color, or note.
• understrege de vigtigste sætninger specifically means drawing a line under the most important sentences.

So understrege is more specific than markere.

Because the sentence means new words in a general sense, not the new words as a specific known group.

So:

• markere nye ord = mark new words
• markere de nye ord = mark the new words

The version without de is more general and fits the meaning of a reading habit.

Because Danish often uses the present tense for general habits and repeated actions, just like English does.

• Når jeg læser en svær tekst = When I read a difficult text

This does not mean only right now. It means whenever this happens or in situations where I read a difficult text.

So the present tense here expresses a regular pattern.

Yes. Danish allows different word orders depending on what you want to emphasize.

For example, you could also say:

• Jeg plejer både at markere nye ord og understrege de vigtigste sætninger, når jeg læser en svær tekst.

This version starts with Jeg, so the main clause keeps the normal order:

• Jeg plejer ...

The original version puts the when-clause first, which gives a bit more emphasis to the situation:

• Når jeg læser en svær tekst, plejer jeg ...

Both are natural.