Learn Hardcore German: Unsere frühere Mitbewohnerin war beim Putzen oft unzuverlässig, aber sonst sehr freundlich. - Our former roommate was often unreliable when cleaning but otherwise very friendly.

You could also say unsere ehemalige Mitbewohnerin; in this context, frühere and ehemalige are practically interchangeable and both sound natural.

Very subtle nuance (not important for basic use):

frühere can sound a bit more neutral or chronological (earlier in time),
ehemalige can sound slightly more official or “ex‑”, as in ehemaliger Chef (former boss).

In everyday speech about a roommate, both are fine.

Why is it unsere frühere Mitbewohnerin and not unser frühere Mitbewohnerin?

Because Mitbewohnerin is feminine and in the nominative case (it’s the subject of the sentence).

Feminine nominative singular:
- definite article: die Mitbewohnerin
- possessive with wir: unsere Mitbewohnerin

So:

unsere frühere Mitbewohnerin = nominative feminine singular
(unsere and frühere both take the -e ending to match Mitbewohnerin).

If the noun were masculine, you’d have:

unser früherer Mitbewohner (masculine nominative singular: früherer, no extra -e on unser).

Why Mitbewohnerin and not Mitbewohner? What’s the difference?

Mitbewohner = (male) roommate / flatmate
Mitbewohnerin = (female) roommate / flatmate

The ending -in usually marks a female person:

Lehrer (male teacher) → Lehrerin (female teacher)
Mitbewohner → Mitbewohnerin

So unsere frühere Mitbewohnerin explicitly says the roommate was female.
If it were a man, you’d say unser früherer Mitbewohner.

What does beim Putzen mean, and how is it formed?

Beim Putzen literally means “while cleaning / when cleaning / in the act of cleaning.”

It’s made of:

bei dem → contracted to beim (this is very common in German)
Putzen = a noun made from the verb putzen (“to clean”)

So:

bei dem Putzen = “during the cleaning / at the cleaning” → beim Putzen

In English we use a gerund (cleaning).
In German, the verb is turned into a noun (capitalized, with -en) after a preposition:

beim Lesen (while reading)
beim Kochen (while cooking)
beim Putzen (while cleaning)

Why is Putzen capitalized?

Because here Putzen is not used as a verb but as a noun: das Putzen = “the cleaning.”

After a preposition like bei, German often uses a noun form:

beim Putzen (while cleaning)
nach dem Essen (after eating / after the meal)
vor dem Schlafengehen (before going to sleep)

All nouns in German must be capitalized, so Putzen starts with a capital P.

Which case is used in beim Putzen, and why?

The case is dative.

bei always takes the dative case.
bei dem Putzen → contracted to beim Putzen

You can see the dative in dem (masculine/neuter dative singular article).
So the underlying form is bei dem Putzen (dative) → beim Putzen.

What does oft do in the sentence, and could it be in a different position?

Oft means “often” and modifies war unzuverlässig (“was unreliable”).

Standard, natural word order here:

Unsere frühere Mitbewohnerin war beim Putzen oft unzuverlässig…

You could move oft, but you must keep the verb in second position:

Unsere frühere Mitbewohnerin war oft beim Putzen unzuverlässig… (focus more on the frequency)
Oft war unsere frühere Mitbewohnerin beim Putzen unzuverlässig… (starts with “Often …” for emphasis)

All three versions are grammatically correct, but the original one is very typical and neutral.

What does aber sonst mean together, and can I just say aber?

Aber sonst here means “but otherwise / but apart from that.”

So the sentence is:

She was often unreliable when cleaning, but otherwise very friendly.
aber = but
sonst = otherwise / in other respects

If you say only aber:

… war beim Putzen oft unzuverlässig, aber sehr freundlich.

This just means:

“…was often unreliable when cleaning but very friendly.”

Without sonst, the idea “in all other respects” is weaker.
Aber sonst emphasizes the contrast between that one negative aspect (unreliable at cleaning) and everything else (very friendly).

What’s the difference between unzuverlässig and nicht zuverlässig?

Both can mean “unreliable”, but:

unzuverlässig = the standard, single-word adjective for “unreliable”
nicht zuverlässig = literally “not reliable”

In this sentence, unzuverlässig is the most natural choice.
You could say:

Sie war beim Putzen oft nicht zuverlässig, but it sounds a bit more roundabout.
Unzuverlässig is more direct and idiomatic, especially in written German.

Why is the verb war (simple past) used instead of ist … gewesen (present perfect)?

War is the simple past (Präteritum) of sein (“to be”).

In spoken German, people often prefer the present perfect:

Sie ist beim Putzen oft unzuverlässig gewesen…

However, for “sein” and “haben”, the simple past (war, hatte) is still very common and natural, even in everyday speech, and especially in written German.

So:

Sie war beim Putzen oft unzuverlässig… = natural, slightly more literary/written style.
Sie ist beim Putzen oft unzuverlässig gewesen… = also correct, somewhat heavier, more explicit about the completed past.

In a sentence like this, war is the most typical choice.