Learn Hardcore German: Die Mülltonne stinkt im Sommer, obwohl sie regelmäßig gereinigt wird. - The trash bin stinks in summer, although it is cleaned regularly.

German nouns have grammatical gender that usually has to be memorized.
Mülltonne happens to be feminine, so it takes the article die in the nominative singular:

die Mülltonne – the trash can

There is no logical reason tied to meaning; it’s just how the word is defined in German. You always learn new nouns together with their article: die Mülltonne, der Tisch, das Auto, etc.

Why is it stinkt and not something like riecht schlecht?

Both are possible, but they differ in tone:

stinken = “to stink,” “to reek” – a strong, clearly negative word
schlecht riechen = “to smell bad” – somewhat softer, more neutral

In this sentence, stinkt emphasizes that the smell is really unpleasant.
You could say:

Die Mülltonne stinkt im Sommer … (strong, natural phrasing)
Die Mülltonne riecht im Sommer schlecht … (correct, but less vivid)

Everyday German often prefers stinken for trash, garbage, dirty clothes, etc.

What case is die Mülltonne in here, and why?

Die Mülltonne is in the nominative case because it is the subject of the sentence—the thing that does the action:

Die Mülltonne (subject) stinkt (verb) …

In basic main clauses, the subject is usually in the nominative case and appears in or near the first position in the sentence.

Why is it im Sommer and not in dem Sommer?

im is simply the contracted form of in dem:

in dem Sommer → im Sommer

Both are grammatically correct, but in normal speech and writing, Germans almost always use the contraction im.

Also, Sommer here is in the dative case (after the preposition in indicating time), so dem is the correct article to contract with: in dem Sommer → im Sommer.

Does im Sommer mean “in summer in general” or “this summer”?

By default, im Sommer means in summer (generally, every summer).
Context could make it mean “this summer,” but if you clearly want to say this specific summer, you’d usually add a word:

in diesem Sommer – this summer
letzten Sommer – last summer

As written, the sentence most naturally describes what typically happens in summer.

Why is there a comma before obwohl?

In German, you must put a comma before most subordinating conjunctions, such as:

obwohl, weil, dass, wenn, als, während, bevor, nachdem, etc.

obwohl starts a subordinate clause (obwohl sie regelmäßig gereinigt wird), so a comma is required:

Main clause: Die Mülltonne stinkt im Sommer,
Subordinate clause: obwohl sie regelmäßig gereinigt wird.

What is the function of obwohl in this sentence?

obwohl means “although / even though” and introduces a contrasting fact:

Fact: The trash can stinks.
Contrasting fact: It is cleaned regularly.

obwohl links these two ideas to show that the second one is unexpected in relation to the first:

Die Mülltonne stinkt im Sommer, obwohl sie regelmäßig gereinigt wird.
“The trash can stinks in summer, even though it is cleaned regularly.”

Why is the verb at the end of the clause: obwohl sie regelmäßig gereinigt wird and not obwohl sie wird regelmäßig gereinigt?

In German, subordinate clauses (introduced by words like obwohl, weil, dass, wenn, weil) send the conjugated verb to the end of the clause.

So the order is:

obwohl (subordinating conjunction)
sie (subject)
regelmäßig (adverb)
gereinigt (past participle)
wird (conjugated verb – must be at the end)

→ obwohl sie regelmäßig gereinigt wird

obwohl sie wird regelmäßig gereinigt is wrong in standard German because the conjugated verb (wird) is not in the final position.

What exactly is wird gereinigt? Why not just man reinigt sie?

wird gereinigt is the present passive:

sie wird gereinigt = “it is cleaned” / “it gets cleaned”

Structure:

wird – present tense of werden
gereinigt – past participle of reinigen

Passive is used here because we care about the state and action on the trash can, not about who does the cleaning.

You could say it actively:

… obwohl man sie regelmäßig reinigt. – “although people regularly clean it.”

Both are correct. The passive just sounds natural here because the agent (“who cleans it”) is unimportant.

Why is sie used for “the trash can”? Does it mean “she”?

sie here refers back to die Mülltonne, which is grammatically feminine.
In German, pronouns agree with grammatical gender, not with natural gender or English “it/he/she” distinctions:

die Mülltonne → sie (“it,” referring to the trash can)
der Tisch → er (“it,” referring to the table)
das Auto → es (“it,” referring to the car)

So in English you translate sie here as “it,” not “she.”

Why is the order sie regelmäßig gereinigt wird and not sie gereinigt regelmäßig wird?

In subordinate clauses, the conjugated verb goes last.
The elements before it (subject, objects, adverbs, participles) can move somewhat, but some orders are much more natural.

Here:

sie – subject
regelmäßig – adverb (how often)
gereinigt – participle
wird – conjugated verb (must be final)

→ sie regelmäßig gereinigt wird sounds natural and clear.

Sie gereinigt regelmäßig wird is grammatically weird and unidiomatic, even if a native speaker can guess the meaning.

What is the difference between gereinigt, gewaschen, and geputzt?

All three can relate to “cleaning,” but they’re used differently:

reinigen / gereinigt – to clean (more neutral, often used in written language or for surfaces, containers, technical things)
waschen / gewaschen – to wash with water (and usually soap); used for clothes, hands, hair, cars, etc.
putzen / geputzt – to clean by wiping, scrubbing, brushing; often for rooms, windows, teeth.

For a Mülltonne, all three can appear in context, but:

gereinigt works well and sounds a bit neutral/technical.
gewaschen focuses on washing it out with water.
geputzt on scrubbing/wiping it.

regelmäßig gereinigt wird is very natural here and emphasizes the regular cleaning process in general.

Why is gereinigt used and not a form like reinigt wird?

In the werden-passive, you always use:

werden (conjugated) + past participle of the main verb

For reinigen, the past participle is gereinigt:

sie wird gereinigt – it is cleaned
sie wurde gereinigt – it was cleaned
sie ist gereinigt worden – it has been cleaned

You cannot say sie wird reinigt; that mixes present tense and participle wrongly.
Correct is always wird gereinigt, wurde gereinigt, etc.

Can I replace obwohl with trotzdem? For example: Die Mülltonne stinkt im Sommer, sie wird trotzdem regelmäßig gereinigt.

Yes, but it changes the structure:

obwohl introduces a subordinate clause:
- Die Mülltonne stinkt im Sommer, obwohl sie regelmäßig gereinigt wird.
trotzdem is an adverb meaning “nevertheless / despite that”; it starts (or appears near the start of) a new main clause:
- Die Mülltonne stinkt im Sommer. Sie wird trotzdem regelmäßig gereinigt.
- Or: Die Mülltonne stinkt im Sommer, sie wird trotzdem regelmäßig gereinigt. (two main clauses joined by a comma – more casual speech style)

Meaning is similar (“despite that”), but obwohl ties the two parts more tightly in one sentence.

Can the German present tense here also mean “stinks / is stinking / will stink”?

Yes. German Präsens is quite flexible. In this sentence, it most naturally expresses a general or regular situation:

Die Mülltonne stinkt im Sommer …
- “The trash can stinks in summer.” (every summer / typically)

Depending on context, German Präsens can correspond to:

English simple present: “stinks”
Present progressive: “is stinking” (if talking about now)
Near future: “will stink” or “is going to stink” (with a time expression)

Here, with im Sommer, it’s best understood as a general, regular occurrence, like a habit or pattern.