Die Hündin wedelt mit dem Schwanz, sobald meine Schwester nach Hause kommt.

Hündin means female dog. The basic word Hund can mean dog in general or male dog depending on context.

Yes, Hündin is a normal, neutral everyday word in German. Unlike the English word bitch, it is not automatically rude or insulting.

Also, die is used because Hündin is a feminine noun.

Both verbs are in the 3rd person singular present tense.

• die Hündin wedelt = the female dog wags
• meine Schwester kommt = my sister comes

In the present tense, many German verbs take -t with er / sie / es.

• ich wedle
• du wedelst
• sie wedelt

The same pattern appears with kommen:

• ich komme
• du kommst
• sie kommt

Because mit always takes the dative case.

The noun is:

• der Schwanz = nominative
• dem Schwanz = dative

So:

• mit dem Schwanz = with the tail

German normally says mit dem Schwanz wedeln for to wag its tail.

Sobald means as soon as. It emphasizes that one action happens immediately after another.

So here, the idea is: the dog starts wagging its tail as soon as the sister comes home.

By contrast, wenn usually means when, whenever, or sometimes if, depending on context. If you used wenn here, it would sound a bit less immediate and more like a general repeated situation.

• sobald meine Schwester nach Hause kommt = as soon as my sister comes home
• wenn meine Schwester nach Hause kommt = when / whenever my sister comes home

Because sobald introduces a subordinate clause, and in German subordinate clauses the conjugated verb goes to the end.

So:

• main clause: Die Hündin wedelt mit dem Schwanz
• subordinate clause: sobald meine Schwester nach Hause kommt

That is why German says:

• ... sobald meine Schwester nach Hause kommt

not:

• ... sobald meine Schwester kommt nach Hause

Because German uses a comma to separate a main clause from a subordinate clause.

Since sobald meine Schwester nach Hause kommt is a subordinate clause, the comma is required.

This is very common in German:

• Ich gehe, weil ich müde bin.
• Sie lacht, wenn er singt.
• Die Hündin wedelt ..., sobald meine Schwester nach Hause kommt.

Because German distinguishes between movement toward home and being at home.

• nach Hause = home / to home → movement
• zu Hause = at home → location

So:

• Meine Schwester kommt nach Hause. = My sister comes home.
• Meine Schwester ist zu Hause. = My sister is at home.

This is a fixed expression, so learners should just memorize it as a pair:

• nach Hause = going home
• zu Hause = being at home

Because meine Schwester is the subject of the subordinate clause — she is the one doing the action kommt.

Subjects are in the nominative case.

So:

• meine Schwester = nominative singular feminine

The possessive word meine matches the gender, number, and case of Schwester.

Yes. You can also say:

Sobald meine Schwester nach Hause kommt, wedelt die Hündin mit dem Schwanz.

That is completely correct.

The important thing is that in German the finite verb in the main clause must still come in second position. When the subordinate clause comes first, it counts as the first element, so the verb comes immediately after it:

• Sobald meine Schwester nach Hause kommt, wedelt die Hündin ...

not:

• Sobald meine Schwester nach Hause kommt, die Hündin wedelt ...

It can describe either, depending on context, but it most naturally sounds like a habitual/repeated action:

Every time the sister comes home, the dog wags its tail.

German often uses the present tense for this kind of regular behavior, just as English does in sentences like:

• My dog gets excited when my sister comes home.

So even though the action happens at a particular moment, the present tense is perfectly normal here.