Wenn der Mülleimer voll ist, leere ich ihn sofort aus.

Wenn introduces a subordinate clause and often means if or when, depending on context.

In this sentence, Wenn der Mülleimer voll ist, leere ich ihn sofort aus, it has a whenever/when sense:

• When the trash can is full, I empty it right away.
• It can also feel a bit like if in English, but here it usually describes a repeated situation or general rule.

A useful rule:

• wenn = when/whenever for repeated or general situations
• als = when for a one-time event in the past

So wenn is the natural choice here.

Because wenn creates a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• der Mülleimer ist voll = main clause word order
• wenn der Mülleimer voll ist = subordinate clause, so ist moves to the end

This is one of the most important German word-order patterns to learn.

Because the sentence begins with a subordinate clause, and that whole clause counts as position 1 in the main clause.

German main clauses normally have the finite verb in position 2. So after:

• Wenn der Mülleimer voll ist, ...

the next part must start with the verb:

• leere ich ihn sofort aus

So the pattern is:

• [Subordinate clause], [verb] [subject] ...

That is why it is leere ich, not ich leere.

Because der Mülleimer is the subject of the subordinate clause Wenn der Mülleimer voll ist.

Ask: What is full?
Answer: der Mülleimer

Since it is the subject, it stays in the nominative case:

• der = masculine nominative

You would use den Mülleimer if it were a direct object, for example:

• Ich sehe den Mülleimer.

Because voll is being used as a predicate adjective, not as an adjective directly before a noun.

Compare:

• der volle Mülleimer = adjective before a noun, so it gets an ending
• der Mülleimer ist voll = predicate adjective after sein, so no ending

That is why the sentence has:

• der Mülleimer voll ist

not something like volle.

Ihn is the accusative pronoun for he/it referring to a masculine noun.

Here it refers back to der Mülleimer, which is masculine:

• der Mülleimer → ihn

Why accusative? Because it is the direct object of ausleeren:

• I empty it

So:

• nominative: er
• accusative: ihn

Even though English says it, German pronouns follow the noun’s grammatical gender, so Mülleimer becomes ihn.

Because ausleeren is a separable verb.

Its two parts are:

• aus- = separable prefix
• leeren = verb

In a normal main clause, the conjugated part goes in the usual verb position, and the prefix moves to the end:

• Ich leere ihn aus.

That is exactly what happens here:

• leere ich ihn sofort aus

Other common separable verbs work the same way:

• Ich räume auf.
• Er steht auf.
• Wir kaufen ein.

But in infinitives or subordinate clauses, the parts stay together:

• Ich will ihn ausleeren.
• ..., weil ich ihn ausleere.

Yes, though in many everyday situations they are very close.

• leeren = to empty
• ausleeren = to empty out

In this sentence, ausleeren sounds very natural for something like a trash can, container, bag, or box. It emphasizes removing the contents.

You may also hear:

• den Mülleimer leeren
• den Mülleimer ausleeren

Both are understandable, but ausleeren often feels especially natural when talking about taking everything out.

Yes, adverbs like sofort can sometimes move around, but some positions sound more natural than others.

Here:

• leere ich ihn sofort aus

is very natural.

The basic idea is:

• conjugated verb early in the clause
• object pronoun often comes fairly early
• adverbs like sofort often come before the separable prefix at the end

Other possible versions exist, but they may sound more marked or emphasize something different. The given word order is a very standard, natural choice.

Because German very often forms compound nouns by joining words together.

Here:

• Müll = trash/garbage
• Eimer = bucket/bin

So Mülleimer is a compound noun. German normally writes these as one word, not as separate words.

This is extremely common in German:

• Haus + Tür = Haustür
• Schlaf + Zimmer = Schlafzimmer
• Wasser + Flasche = Wasserflasche

Also notice that all nouns are capitalized, so Mülleimer begins with a capital letter.

No, that is not correct standard German.

After the initial wenn-clause, the main clause must still follow the verb-second rule. Since the subordinate clause already takes the first position, the conjugated verb comes next:

• Wenn der Mülleimer voll ist, leere ich ihn sofort aus. ✅

Not:

• Wenn der Mülleimer voll ist, ich leere ihn sofort aus. ❌

This is one of the most common word-order mistakes for English speakers, because English does not force this kind of inversion after an opening clause.