Learn Hardcore German: Ich lerne mit meiner Kommilitonin in der Bibliothek. - I study with my fellow student in the library.

lernen = to study/learn in the sense of doing homework, revising, practicing, preparing for a test.
- Ich lerne in der Bibliothek. = I am studying (doing work) in the library.
studieren = to study in the sense of being enrolled in a program at a university or majoring in something.
- Ich studiere Physik. = I study physics (as my degree).

In your sentence, you are describing the activity of sitting and working in the library, so lernen is correct.

What exactly does Kommilitonin mean, and when would I use it?

Kommilitonin means female fellow student at a university or similar higher-education institution, usually someone in the same program or at the same university.

Some nuances:

It is not just any classmate in school; for school pupils you would more often say Mitschülerin / Mitschüler.
It implies a shared university context, often the same degree program or at least the same university.
It sounds a bit more formal or academic than Mitstudentin, but both are used.

So meine Kommilitonin = my fellow student (female) at university.

Why is it Kommilitonin and not Kommilitone here?

German marks grammatical gender on many nouns:

der Kommilitone = male fellow student
die Kommilitonin = female fellow student

In your sentence, Kommilitonin shows the person is female.
If the person were male, you’d say:

Ich lerne mit meinem Kommilitonen in der Bibliothek.

For a mixed or unspecified group you might see:

Kommilitonen (masculine plural, often used generically)
Or gender-inclusive forms in writing, e.g. Kommiliton*innen, Kommiliton:innen, etc.

Why do we say mit meiner Kommilitonin and not mit meine Kommilitonin?

Because mit always takes the dative case.

mit → always dative
Kommilitonin is feminine singular: die Kommilitonin in the nominative.
Feminine singular dative with a mein- word needs the ending -er:

Nominative: meine Kommilitonin (subject)
Accusative: meine Kommilitonin (direct object)
Dative: meiner Kommilitonin (after mit, “with my fellow student”)

So mit meiner Kommilitonin is “with my (female) fellow student” in the dative case.

What case is meiner Kommilitonin, and how can I recognize it?

meiner Kommilitonin is dative feminine singular.

You can recognize it by:

The preposition mit, which always requires dative.
The ending -er on meiner, which is typical for dative feminine singular with possessive determiners (meiner Mutter, meiner Freundin, meiner Lehrerin).

Pattern (singular, using mein-) for a feminine noun:

Nominative: meine Kommilitonin
Accusative: meine Kommilitonin
Dative: meiner Kommilitonin
Genitive: meiner Kommilitonin (less frequent in speech)

Here, the context (after mit) makes clear it must be dative.

Why is it in der Bibliothek and not in die Bibliothek?

The preposition in can take either dative or accusative, depending on the meaning:

in
- dative → location, “where something is” (no movement into it)
  - Ich lerne in der Bibliothek. = I am studying in the library (location).
in
- accusative → direction, “where something is going (to)”
  - Ich gehe in die Bibliothek. = I am going into the library (movement).

Your sentence describes where the studying happens, not a movement into the library, so dative is used: in der Bibliothek.

Why is it der Bibliothek in the dative, when the dictionary says die Bibliothek?

The dictionary usually lists nouns in the nominative singular:

Nominative: die Bibliothek (the library)

But German articles change with case. For a feminine noun like Bibliothek, the definite article becomes der in the dative:

Nominative: die Bibliothek (subject, basic form)
Accusative: die Bibliothek (direct object)
Dative: der Bibliothek (after many prepositions like in, mit, bei when they require dative)
Genitive: der Bibliothek

Because in here expresses location and therefore takes dative, you get in der Bibliothek.

Can I change the word order and say Ich lerne in der Bibliothek mit meiner Kommilitonin?

Yes, that is correct German.

Both are fine:

Ich lerne mit meiner Kommilitonin in der Bibliothek.
Ich lerne in der Bibliothek mit meiner Kommilitonin.

The meaning is essentially the same. You’re just changing the order of the adverbial phrases (“with whom?” and “where?”). German allows relatively free order for these, though the most neutral order is often:

Time – Manner – Place (not a strict rule, but a common guideline)

In this sentence, both variants sound natural.

Does Ich lerne mean “I learn” or “I am learning / I am studying” in English?

It can mean both, depending on context. German has only one present tense form:

Ich lerne can translate as:
- I learn (habit, general ability)
- I am learning / I am studying (right now, ongoing action)

In everyday conversation, Ich lerne in der Bibliothek is usually understood as “I’m studying in the library (now / these days).” The exact nuance comes from context, not from verb form.

Can I drop Ich and just say Lerne mit meiner Kommilitonin in der Bibliothek?

In a normal statement, no. German usually requires an explicit subject pronoun:

Standard: Ich lerne mit meiner Kommilitonin in der Bibliothek.
Lerne mit meiner Kommilitonin in der Bibliothek. → sounds like an imperative (“Study with my fellow student in the library”), and even then German would normally need a form like Lern(e) for du or Lernt for ihr.

Dropping Ich is not natural in standard German main clauses. You should keep Ich.

Why do we use mit and not bei or something else?

Because in this sentence you are expressing “together with someone”:

mit = with (together with someone/something)
- Ich lerne mit meiner Kommilitonin. = I study with my fellow student.

bei is different:

bei = at / near / with (in the sense of “at someone’s place” or “employed by”)
- Ich lerne bei meiner Mutter. = I study at my mother’s place / with my mother present.
- Ich arbeite bei Siemens. = I work at Siemens.

To express “together with a person” as in a partner in the activity, mit is the standard choice.

How would I say “with my classmates” instead of “with my fellow student”?

A few options, depending on context:

At university:

mit meinen Kommilitoninnen = with my (female) fellow students
mit meinen Kommilitonen = with my (male or mixed group) fellow students

In school:

mit meinen Mitschülerinnen = with my (female) classmates
mit meinen Mitschülern = with my (male or mixed) classmates

Notice the changes:

Plural meine → meinen in the dative plural after mit:
- Nominative plural: meine Kommilitonen / Mitschüler
- Dative plural: meinen Kommilitonen / Mitschülern (with -n on the noun if possible)

Why are Kommilitonin and Bibliothek capitalized?

In German, all nouns are capitalized, no matter where they appear in the sentence:

die Kommilitonin → Kommilitonin
die Bibliothek → Bibliothek

Adjectives, verbs, and most other words are not capitalized in the middle of sentences, but nouns always are. That’s why you see Ich lerne mit meiner Kommilitonin in der Bibliothek.

How is meiner formed, and what are the other forms of mein?

mein works like a possessive determiner (“my”) and takes endings similar to the indefinite article ein.

For a typical feminine noun like Kommilitonin (singular):

Nominative: meine Kommilitonin (my fellow student – subject)
Accusative: meine Kommilitonin (my fellow student – direct object)
Dative: meiner Kommilitonin (with my fellow student – after mit)
Genitive: meiner Kommilitonin

In your sentence:

Preposition mit → dative
Feminine singular dative → meiner Kommilitonin

So meiner is just the dative feminine singular form of mein.

How would I say this sentence in the past, like “I studied with my fellow student in the library”?

The natural German past of lernen in spoken language is the Perfekt (haben + Partizip II):

Ich habe mit meiner Kommilitonin in der Bibliothek gelernt.

Structure:

Ich (subject)
habe (auxiliary verb in 2nd position)
adverbials: mit meiner Kommilitonin in der Bibliothek
gelernt (past participle at the end)

This means: I studied / I was studying with my fellow student in the library.