Learn Hardcore German: Meine Schwester schnarcht manchmal laut, wenn sie sehr müde ist. - My sister sometimes snores loudly when she is very tired.

In German, the subject is in the nominative case.
Feminine nominative singular with the possessive mein- takes the ending -e → meine Schwester (my sister).

Meiner Schwester would be:

dative feminine (e.g. Ich helfe meiner Schwester. – I help my sister.)
or genitive feminine in some contexts (more formal/literary).

Since meine Schwester is doing the action (snoring), it must be nominative: meine, not meiner.

Why is the verb schnarcht in second position, and can I move manchmal before it?

German main clauses follow the V2 rule: the conjugated verb must be in second position.

Meine Schwester = 1st element (the whole phrase counts as one slot)
schnarcht = verb, must be in 2nd position
manchmal laut = everything after that

So:

Meine Schwester schnarcht manchmal laut ... is correct.

You can move manchmal to the beginning:

Manchmal schnarcht meine Schwester laut, wenn sie sehr müde ist.

Now:

Manchmal = 1st element
schnarcht = 2nd position (still correct)
meine Schwester laut = rest

What you cannot do in a normal statement is:

*Meine Schwester manchmal schnarcht laut ... (verb is not in 2nd position → wrong).

Where do adverbs like manchmal, laut, and sehr normally go in a German sentence?

There are two things to know:

Position in the clause
- manchmal (time/frequency) usually comes towards the front of the middle field, often right after the verb or subject:
  - Meine Schwester schnarcht manchmal laut ...
- laut (manner: how?) typically comes after frequency/time adverbs:
  - ... schnarcht manchmal laut ...
Order of different kinds of adverbials
A common rule is Time – Manner – Place (TE-KA-MO-LO):
- Time/frequency: manchmal
- Manner: laut
- Place: (not present here)

So manchmal laut is a natural order: sometimes (time) loudly (manner).

sehr is a bit different: it is not a separate adverbial, it is an intensifier attaching directly to müde:

sehr müde = very tired (functions as one adjective phrase inside the wenn-clause).

Why is laut not laute or lauter here?

laut here is used adverbially: it describes how she snores (loudly), not what kind of noun.

In German:

Adjectives before a noun get endings:
- laute Musik (loud music)
- ein lauter Schnarcher (a loud snorer)
Adjectives used after a verb as an adverb or predicate usually have no ending:
- Sie schnarcht laut. (She snores loudly.)
- Er spricht leise. (He speaks quietly.)

So laut is the correct base form here.
Laute or lauter would suggest it’s directly modifying a noun, which it does not.

Why is there a comma before wenn, and why does ist go at the end of wenn sie sehr müde ist?

wenn introduces a subordinate clause (Nebensatz). In German:

A subordinate clause is normally separated from the main clause by a comma:
- Meine Schwester schnarcht manchmal laut, wenn sie sehr müde ist.
In a subordinate clause introduced by a subordinating conjunction like wenn, the conjugated verb goes to the end:
- wenn (conjunction)
- sie (subject)
- sehr müde (predicate/adverb)
- ist (conjugated verb at the end)

So the verb order difference is:

Main clause: Meine Schwester schnarcht manchmal laut ... (verb in 2nd position)
Sub clause: ..., wenn sie sehr müde ist. (verb at the end)

What is the difference between wenn and als in sentences like this?

Both can be translated as when, but they are used differently:

wenn is used for:
- repeated events: whenever, every time that
- general conditions: if/when
als is used for:
- a single event in the past: when (that one time / that period)

In your sentence:

Meine Schwester schnarcht manchmal laut, wenn sie sehr müde ist.

This describes a repeated, general situation (every time she is very tired).
Therefore wenn is correct.

If you were talking about one specific time in the past, you could use als, e.g.:

Als sie gestern sehr müde war, hat sie laut geschnarcht.
(When she was very tired yesterday, she snored loudly.)

Why do we use sie in the second part instead of repeating meine Schwester?

German works very much like English here:

You normally replace a previously mentioned noun phrase with a pronoun to avoid repetition.

So instead of:

Meine Schwester schnarcht manchmal laut, wenn meine Schwester sehr müde ist. (correct, but clumsy)

you say:

Meine Schwester schnarcht manchmal laut, wenn sie sehr müde ist.

Here, sie = she clearly refers back to meine Schwester, which is feminine singular.
Using sie is more natural and avoids unnecessary repetition.

Why is it sie sehr müde ist and not sie ist sehr müde in the wenn-clause?

This is because of the word order rule in subordinate clauses:

In main clauses: verb in 2nd position
- Sie ist sehr müde.
In subordinate clauses with wenn, dass, weil, etc.:
the conjugated verb goes to the end.
- ..., wenn sie sehr müde ist.

So the clause wenn sie sehr müde ist uses the same elements as sie ist sehr müde, but with the verb moved to the final position because of wenn.

Could we say ..., wenn sie sehr müde wird instead of ist? Would that change the meaning?

Yes, you can say it, but the meaning changes slightly:

... wenn sie sehr müde ist.
- Focus on the state: when she is (already) very tired.
- The snoring happens while she is in that condition.
... wenn sie sehr müde wird.
- Focus on the process of becoming tired: when she becomes very tired / as she gets very tired.
- Suggests the snoring starts during the process of getting tired, not just in the state of being tired.

In most cases describing a typical situation like this, ist (state) is more natural.

Can this sentence also be said with a different word order, like Manchmal schnarcht meine Schwester laut, wenn sie sehr müde ist? Does that change the emphasis?

Yes, that word order is perfectly correct:

Manchmal schnarcht meine Schwester laut, wenn sie sehr müde ist.

Differences:

Meine Schwester schnarcht manchmal laut, ...
- Slight emphasis on meine Schwester (who is it that snores? my sister).
- manchmal is in the middle; frequency is mentioned, but not fronted.
Manchmal schnarcht meine Schwester laut, ...
- Emphasis on manchmal (how often / the fact that it happens).
- The sentence starts with the idea of sometimes.

The meaning is essentially the same; it is mainly a matter of focus or what you want to highlight first.

Is sehr müde an adjective phrase like in English, and are there any agreement endings missing?

Yes, sehr müde is an adjective phrase, similar to English very tired:

müde = adjective (tired)
sehr = adverb/intensifier (very), modifying müde

There are no endings missing. In German:

Adjectives before a noun take endings:
- eine müde Schwester (a tired sister)
- meine sehr müde Schwester
Adjectives used after the verb (predicative adjectives) usually have no ending:
- Sie ist müde.
- Sie ist sehr müde.

So sie ist sehr müde and inside the clause wenn sie sehr müde ist are both fully correct without extra endings.

How would the sentence change if I talked about more than one sister?

You would need to change everything to the plural:

Meine Schwestern schnarchen manchmal laut, wenn sie sehr müde sind.

Changes:

Subject plural
- Meine Schwester → Meine Schwestern (my sisters)
Verb plural in the main clause
- schnarcht (3rd person singular) → schnarchen (3rd person plural)
Verb plural in the subordinate clause
- ist (3rd person singular) → sind (3rd person plural)
sie stays the same in spelling, but now means they instead of she, because its antecedent is plural Schwestern.

So the structure is the same, but subject and verbs agree in plural.