Learn Hardcore German: Meine Mitbewohnerin erzählt mir abends immer etwas Interessantes aus ihrem Studium. - My female roommate always tells me something interesting from her studies in the evening.

Mitbewohnerin is grammatically feminine (ending -in usually marks feminine people).
It is the subject of the sentence, so it’s in the nominative case.
Feminine nominative with mein- takes the ending -e → meine Mitbewohnerin.

Comparison (nominative singular):

mein Mitbewohner (my male flatmate – masculine)
meine Mitbewohnerin (my female flatmate – feminine)
mein Studium (my degree course – neuter)

What exactly does Mitbewohnerin mean, and how is it different from Mitbewohner?

Mitbewohnerin is a compound noun:

mit = with
Bewohner = inhabitant
Mitbewohner = person you live with (flatmate/roommate), usually male or gender‑neutral in colloquial use
Mitbewohnerin = explicitly a female flatmate/roommate (the -in marks feminine).

Plural:

die Mitbewohner (mixed group or all male)
die Mitbewohnerinnen (all female)

In your sentence, the person is explicitly female, hence Mitbewohnerin.

Why is it erzählt mir and not erzählt mich?

The verb erzählen works like “to tell (someone) something”:

person who receives the information → dative
thing that is told → accusative

So:

jemandem etwas erzählen (to tell someone something)
mir = dative of ich
- ich → mich (accusative), mir (dative)

In your sentence:

mir (to me) = indirect object, dative
etwas Interessantes (something interesting) = direct object, accusative

So erzählt mir is correct: “(she) tells me (something)” in the German case system.

What does abends mean, and how is it different from am Abend?

abends is an adverb meaning “in the evenings / in the evening (regularly, habitually)”.

abends = repeatedly, habitually in the evening, on most evenings
- Sie liest abends. = She reads in the evenings (as a routine).
am Abend = “in the evening” but more like a specific or more concrete time frame
- Wir sehen uns am Abend. = We’ll see each other this evening / in the evening (one occasion).

In your sentence, abends fits because it’s describing a regular habit:

…erzählt mir abends immer… = she always tells me (things) in the evenings (as a routine).

Could the word order be Meine Mitbewohnerin erzählt abends mir immer…?

That word order is grammatically possible, but it sounds odd and unnatural.

Basic “middle field” order rules in German:

Subject and verb: Meine Mitbewohnerin erzählt
Pronoun objects before full noun objects
Time–manner–place often used for adverbs, but pronouns usually come earlier.

In your sentence the natural order is:

Meine Mitbewohnerin erzählt mir abends immer etwas Interessantes aus ihrem Studium.

Here:

mir (pronoun, dative) comes early.
abends, immer (adverbs)
etwas Interessantes (accusative object)
aus ihrem Studium (prepositional phrase)

Meine Mitbewohnerin erzählt mir immer abends … is also possible (slight change of emphasis), but erzählt abends mir is marked and unusual.

Why is it abends immer and not immer abends? Are both correct?

Both orders are grammatically correct; they just sound slightly different:

abends immer – a bit more focus on the time frame “in the evenings”
- Roughly: “in the evenings, she always …”
immer abends – a bit more focus on “always” as in “every single evening”
- Roughly: “always in the evening (never in the morning, etc.) …”

Native speakers would most often say either:

Meine Mitbewohnerin erzählt mir abends immer …
Meine Mitbewohnerin erzählt mir immer abends …

The difference is subtle and often just stylistic. Your original abends immer is perfectly natural.

What is etwas here – a pronoun or an adverb – and what does it mean?

Here etwas is an indefinite pronoun meaning “something”.

etwas Interessantes = “something interesting” (some interesting thing)

Compare:

Ich habe etwas gegessen. = I ate something.
Sie erzählt mir etwas. = She tells me something.

Note: etwas can also be an adverb meaning “a bit / somewhat”, e.g.
Es ist etwas kalt. = It’s a bit cold.
But in your sentence it’s clearly a pronoun (“something”).

Why is it etwas Interessantes and not etwas Interessant?

Because Interessantes is a nominalized adjective (an adjective used as a noun) referring to an unspecified neuter thing.

Pattern:

etwas Gutes = something good
etwas Neues = something new
etwas Interessantes = something interesting

Grammar:

etwas behaves like “no article”, so the adjective takes the strong ending.
The “thing” is neuter singular, and in nominative/accusative neuter singular strong ending is -es:
- gut → Gutes
- neu → Neues
- interessant → Interessantes

Since etwas Interessantes is the direct object, it’s in the accusative, but nominative and accusative neuter look the same here (-es).

Why is Interessantes capitalized?

Because it is a nominalized adjective – an adjective being used as a noun.

General rule:

When an adjective stands alone and functions as a noun, it is capitalized:
- das Gute = the good (thing)
- nichts Neues = nothing new
- etwas Schönes = something beautiful
- etwas Interessantes = something interesting

So Interessantes is capitalized like any other noun.

What case is etwas Interessantes in this sentence?

Accusative singular (neuter).

Reason:

It is what she tells (the “something interesting”), so it’s the direct object of erzählt.
Direct objects in German are normally in the accusative.

Case overview for the key parts:

Meine Mitbewohnerin → subject → nominative
mir → indirect object → dative
etwas Interessantes → direct object → accusative

Why do we use aus ihrem Studium and not, for example, von ihrem Studium?

Both are possible, but they have different nuances:

aus ihrem Studium literally “out of / from her studies”
- Emphasizes that the interesting things come out of her study experiences or content.
- Natural for things that originate in a certain source (studies, work, childhood, etc.).
von ihrem Studium often means “about her studies”
- Focuses more on the topic rather than the source.

Compare:

Sie erzählt mir etwas aus ihrem Studium.
= She tells me something that comes from her studies (e.g. a story, an anecdote, a fact learned in a lecture).
Sie erzählt mir etwas von ihrem Studium.
= She tells me something about her studies (how they are, what she does there, etc.).

In your sentence, aus ihrem Studium nicely expresses “something interesting (that comes) from her studies.”

Why is it ihrem Studium and not ihr Studium or ihren Studium?

Because:

The preposition aus always takes the dative case.
Studium is neuter singular.

So we need dative singular neuter for ihr Studium (“her degree course”).

Declension of ihr- (her) in singular:

Masculine:
- nominative: ihr Vater
- accusative: ihren Vater
- dative: ihrem Vater
Feminine:
- nominative: ihre Mutter
- accusative: ihre Mutter
- dative: ihrer Mutter
Neuter:
- nominative: ihr Studium
- accusative: ihr Studium
- dative: ihrem Studium

Because of aus + dative + neuter:

aus ihrem Studium is the only correct form here.

What exactly does Studium mean here?

Studium is a neuter noun that usually refers to university-level studies or a degree course:

Sie ist im dritten Jahr ihres Studiums.
= She is in the third year of her studies/degree.
Er hat sein Studium abgebrochen.
= He dropped out of university.

It does not mean “school” in general. For that you would use e.g. Schule (school) or Ausbildung (vocational training).

In your sentence, aus ihrem Studium means “from her (university) studies / from her degree program.”

Can the word order at the end be changed, e.g. …erzählt mir abends immer aus ihrem Studium etwas Interessantes?

Yes, German word order in the “middle field” is fairly flexible, but there are preferences.

Natural, most typical version:

…erzählt mir abends immer etwas Interessantes aus ihrem Studium.

Other versions:

…erzählt mir immer abends etwas Interessantes aus ihrem Studium. (also fine)
…erzählt mir abends immer aus ihrem Studium etwas Interessantes. (understandable, but sounds less natural and slightly clumsy)

General tendency:

direct object (etwas Interessantes) usually comes before a longer prepositional phrase (aus ihrem Studium), unless you really want to emphasize the prepositional phrase.

So your original word order is the most idiomatic.