Learn Hardcore German: Sie ist sehr fleißig und oft die erfolgreichste Studentin in ihrer Gruppe. - She is very diligent and often the most successful student in her group.

In German, adjectives that describe the subject and come after sein (and similar verbs like werden, bleiben) are predicate adjectives. Predicate adjectives go after the verb:

Sie ist fleißig. – She is hardworking.

Putting the adjective before the noun is only for adjectives directly attached to a noun:

die fleißige Studentin – the hardworking student

So you say:

Sie ist sehr fleißig. (correct) not
Sie sehr fleißig ist. (wrong in a normal statement)

Why do we say sehr fleißig and not viel fleißig?

Sehr is used to intensify qualities (adjectives and adverbs):

sehr fleißig – very hardworking
sehr schön – very beautiful
sehr schnell – very fast

Viel usually intensifies verbs or quantities:

Sie arbeitet viel. – She works a lot.
Sie hat viel Arbeit. – She has a lot of work.

So:

Sie ist sehr fleißig. – She is very hardworking.
Sie arbeitet viel. – She works a lot.

You normally wouldn’t say viel fleißig.

What exactly does oft modify here, and is its position correct?

In Sie ist sehr fleißig und oft die erfolgreichste Studentin in ihrer Gruppe, oft (“often”) modifies the whole idea of being “the most successful student in her group”.

It means:

She is very hardworking, and she is often the most successful student in her group. (Not necessarily always.)

Position:

oft is placed before the part it belongs to: oft die erfolgreichste Studentin in ihrer Gruppe.
This is natural and correct.

You could also say:

Sie ist sehr fleißig und sie ist oft die erfolgreichste Studentin in ihrer Gruppe.

The meaning is basically the same; the original just avoids repeating ist and sie.

Why is it die erfolgreichste Studentin and not am erfolgreichsten Studentin?

German has two main ways to form the superlative:

Attributive superlative (before a noun):
- die erfolgreichste Studentin – the most successful student
  Structure: Artikel + Adjektivstamm + -st- + Adjektivendung
Predicative superlative (after a verb, no noun right after it):
- Sie ist am erfolgreichsten. – She is the most successful.
  Structure: am + Adjektivstamm + -st- + -en

Because erfolgreichste stands directly before a noun (Studentin), you must use the attributive form:

die erfolgreichste Studentin (correct)
not
am erfolgreichsten Studentin (wrong)

Why is there die before erfolgreichste Studentin?

Die is the definite article for feminine singular in the nominative case.

We use the definite article here because superlatives like erfolgreichste usually refer to a unique, specific person within a group:

die erfolgreichste Studentin in ihrer Gruppe
→ the most successful student in her group (there is only one “most successful”)

Using eine erfolgreichste Studentin would be ungrammatical; superlatives of this type naturally go with the definite article (der / die / das).

How is erfolgreichste formed from erfolgreich, and why does it end in -e?

Steps:

Base adjective: erfolgreich (successful)
Superlative stem: erfolgreichst-
Add adjective ending according to gender, case, and article.

Here we have:

die erfolgreichste Studentin

Studentin is:

feminine
singular
nominative (subject of the second part of the sentence)
with definite article die

With die in feminine nominative, the adjective ending is -e, so:

die erfolgreichste Studentin

Why is the word Studentin used and not Student?

German has separate forms for male and female persons in many professions and roles:

der Student – male student
die Studentin – female student

Here, Studentin tells you explicitly that the person is female. Since the pronoun Sie (here: she) and the possessive ihrer (her) refer to a woman, Studentin is the grammatically and factually consistent choice.

If the student were male, you’d say:

Er ist sehr fleißig und oft der erfolgreichste Student in seiner Gruppe.

What case is ihrer in, and why does it have this ending?

In in ihrer Gruppe, the preposition in is followed by dative here, because it refers to location (“in her group”, not “into her group”).

die Gruppe – feminine noun

Dative singular feminine of the definite article is:

der (e.g. in der Gruppe – in the group)

For the possessive ihr- (“her”), the dative feminine singular ending is -er, so:

in ihrer Gruppe – in her group

So:

Case: dative
Gender/number: feminine singular
Ending: -er → ihrer

Does Sie here mean “she” or formal “you”?

In this sentence, Sie means “she”.

Reason:

The possessive pronoun is ihrer (lowercase i), meaning “her”.
If it were formal “your”, it would be capitalized: Ihrer Gruppe.

So we have:

Sie (she) … in ihrer Gruppe (in her group).

The capitalization at the start of the sentence doesn’t help with the distinction, but the lowercase ihrer shows that it is third-person feminine, not formal “you”.

Why is there no comma before und in this sentence?

In German, you usually do not put a comma before und when it simply connects:

two adjectives: sehr fleißig und freundlich
two parts of a compound predicate: ist sehr fleißig und oft die erfolgreichste Studentin …

Here, ist sehr fleißig and (ist) oft die erfolgreichste Studentin in ihrer Gruppe share the same subject (Sie) and the same verb (ist, which is only written once).

So:

No comma is needed: Sie ist sehr fleißig und oft die erfolgreichste Studentin in ihrer Gruppe.

Could I also say Sie ist sehr fleißig und die erfolgreichste Studentin in ihrer Gruppe without oft? Would that change the meaning?

Yes, you can say:

Sie ist sehr fleißig und die erfolgreichste Studentin in ihrer Gruppe.

Meaning difference:

With oft:
She is often the most successful student – it happens frequently, but not necessarily always.
Without oft:
It sounds more like a stable fact: she is the most successful student in her group (as a general, current status), not just from time to time.

Why is Gruppe in the dative case after in, and not accusative?

The preposition in can take either:

dative → for location (where?)
accusative → for movement (where to?)

Here, in ihrer Gruppe describes a location / membership:

She is (where?) in her group among the other students.

That’s a static location, so dative is used:

in ihrer Gruppe (dative) – in her group

If it described movement into the group, it would be accusative:

Sie geht in ihre Gruppe. – She goes into her group.