Meine Mitbewohnerin putzt das Waschbecken mit einem Schwamm, als würde morgen Besuch kommen.

Mitbewohnerin means female roommate / flatmate.

• Mitbewohner = roommate / someone you live with
• -in = the feminine ending for many nouns referring to people

So:

• mein Mitbewohner = my male roommate
• meine Mitbewohnerin = my female roommate

That is also why the sentence starts with Meine and not Mein: the noun is feminine.

Because Mitbewohnerin is:

• feminine
• in the nominative case
• singular

It is the subject of the sentence, so the possessive determiner must match that:

• mein = masculine/neuter nominative singular
• meine = feminine nominative singular (and also plural)

So meine Mitbewohnerin is the correct form.

German main clauses normally follow the verb-second rule.

In this sentence:

• Meine Mitbewohnerin = position 1
• putzt = position 2

So the basic structure is:

Meine Mitbewohnerin | putzt | das Waschbecken ...

That is standard German word order in a main clause.

Waschbecken is a neuter noun, so its definite article in the nominative and accusative singular is das.

Here it is the direct object of putzen, so it is in the accusative:

• das Waschbecken

Since neuter nouns have das in both nominative and accusative singular, the article does not change here.

Because the preposition mit always takes the dative case.

So:

• der Schwamm = the sponge
• after mit → dative singular masculine
• ein Schwamm becomes einem Schwamm

That is why the phrase is:

mit einem Schwamm = with a sponge

This is a very common thing to memorize in German: mit + dative.

This part means as if ... were coming and expresses an unreal or imagined situation.

German often uses als + Konjunktiv II for this idea:

• als würde morgen Besuch kommen

This suggests that no one has actually said visitors are coming; it only looks that way from how thoroughly she is cleaning.

So the structure signals appearance, impression, or comparison with something hypothetical.

Because würde kommen is a Konjunktiv II form, used here to express something hypothetical or not presented as fact.

Compare:

• morgen kommt Besuch = visitors are coming tomorrow
  → sounds like a real fact
• als würde morgen Besuch kommen = as if visitors were coming tomorrow
  → only an impression, not necessarily true

So würde kommen helps create the as if meaning.

Yes. A very common alternative is:

Meine Mitbewohnerin putzt das Waschbecken mit einem Schwamm, als ob morgen Besuch kommen würde.

This means the same thing.

The difference is mainly structural:

• als würde morgen Besuch kommen
  → finite verb comes right after als
• als ob morgen Besuch kommen würde
  → more typical subordinate-clause word order, with the finite verb at the end

Both are natural. The version with als + inverted Konjunktiv II is a bit more compact.

Because Besuch is often used without an article when it means visitors / company in a general sense.

So:

• Besuch kommt = visitors are coming / company is coming
• Wir bekommen Besuch = we’re having visitors

This is idiomatic German. You do not normally need ein or der here.

If you add an article, the meaning can become more specific or sound different depending on context.

Because the sentence after the comma is a subordinate comparative clause.

German normally separates such clauses with a comma:

• main clause: Meine Mitbewohnerin putzt das Waschbecken mit einem Schwamm
• subordinate/comparative clause: als würde morgen Besuch kommen

So the comma is required.

German word order is flexible, but morgen is naturally placed early here because it is a time expression.

So:

als würde morgen Besuch kommen

sounds very natural.

You could also say:

als würde Besuch morgen kommen

but that is less neutral and places more focus on Besuch first. The original order is the most normal one.

Yes, putzen is very natural here.

• putzen = to clean, especially in everyday situations
• reinigen = to clean, but often more formal, technical, or impersonal

For household cleaning, German speakers usually prefer putzen:

• das Bad putzen
• die Küche putzen
• das Waschbecken putzen

So the sentence sounds natural and idiomatic.

Usually Waschbecken refers to a sink used for washing, especially a bathroom sink.

For a kitchen sink, German often uses:

• Spüle

That said, in some contexts people may still understand Waschbecken broadly as a sink, but the most typical interpretation here is a bathroom sink.