Im Kurs brauche ich neue Wörter nicht nur zu unterstreichen, sondern auch laut vorzulesen.

Im is a contraction of in dem.

• in dem Kurs = im Kurs
• German often contracts in dem to im

Here Kurs is in the dative because it describes a location or setting: in the course / in class.

So:

• im Kurs = in the course / in class

This is because German follows the verb-second rule in main clauses.

The conjugated verb must be in the second position, not necessarily the second word.

Here the sentence begins with Im Kurs, so that takes the first position. The verb brauche must come next, and the subject ich follows it:

• Im Kurs | brauche | ich | ...

If you started with the subject instead, you could also say:

• Ich brauche im Kurs ...

Both are grammatical, but the emphasis is slightly different.

Here brauchen works a bit like need to.

• ich brauche ... zu unterstreichen
• ich brauche ... vorzulesen

This is similar to saying I need to ...

A useful point: in everyday German, many speakers often prefer müssen in positive statements:

• Ich muss neue Wörter ... unterstreichen und vorlesen.

But brauchen + zu + infinitive is still grammatical and understandable. It can sound a bit more formal or stylistically different, depending on the region and context.

No. Here nicht is part of the fixed expression:

• nicht nur ..., sondern auch ...
• not only ..., but also ...

So the sentence is not saying that the speaker does not need to do something. Instead, it means that two things are required:

1. underlining the new words
2. reading them aloud

So nicht belongs to nicht nur, not to a full negation of brauche.

It is a very common paired structure in German:

• nicht nur A, sondern auch B
• not only A, but also B

In this sentence:

• nicht nur zu unterstreichen
• sondern auch laut vorzulesen

So both infinitive phrases are being linked together. German uses this pattern very much like English uses not only ..., but also ...

Neue Wörter is accusative plural.

Why? Because it is the direct object of the actions unterstreichen and vorlesen.

You underline new words. You read new words aloud.

Since Wörter is plural, the plural form here is:

• nominative plural: neue Wörter
• accusative plural: neue Wörter

So the form looks the same in nominative and accusative plural.

German often uses no article with an indefinite plural noun.

So:

• neue Wörter = new words
• die neuen Wörter = the new words

Here the speaker means new words in a general sense, not a specific previously identified set, so no article is needed.

Because both infinitives at the end share the same object:

• neue Wörter
• to underline them
• and to read them aloud

German often places objects before the final infinitive group. So the structure is:

• brauche ich neue Wörter
• nicht nur zu unterstreichen
• sondern auch laut vorzulesen

This is normal German word order. The object is understood with both verbs.

Because vorlesen is a separable verb:

• base form: vorlesen
• prefix: vor-
• main verb: lesen

When a separable verb appears with zu, the zu goes between the prefix and the verb stem:

• vorlesen → vorzulesen
• anzufangen
• aufzuschreiben

So vorzulesen is exactly what you should expect.

Because both are infinitives connected to brauchen:

• zu unterstreichen
• vorzulesen

With brauchen + infinitive, German normally uses zu.

The second verb also has zu, but it is built into the separable form:

• zu unterstreichen
• vorzulesen = vor + zu + lesen

So both infinitives do contain zu.

Here laut means aloud or out loud.

So:

• laut vorlesen = to read aloud

It describes how the words are read. In other contexts, laut can also mean loud, but here the natural English equivalent is aloud.

Because sondern is a coordinating conjunction, and in German it is normally preceded by a comma.

So:

• nicht nur zu unterstreichen, sondern auch laut vorzulesen

The comma helps separate the two coordinated parts:

1. nicht nur zu unterstreichen
2. sondern auch laut vorzulesen

This is standard punctuation.

Yes, in many contexts you could say:

• Im Kurs muss ich neue Wörter nicht nur unterstreichen, sondern auch laut vorlesen.

That would sound very natural.

But the grammar changes slightly:

• with müssen, you use bare infinitives: unterstreichen, vorlesen
• with brauchen, you use zu + infinitive: zu unterstreichen, vorzulesen

So both can express necessity, but the structure is different.