Wir sortieren alte Kleidung aus.

Because the full verb is aussortieren, which is a separable verb.

In a normal main clause, German splits this kind of verb into:

• the conjugated verb: sortieren
• the prefix: aus

So:

• Wir sortieren alte Kleidung aus.

This is very common in German.

Compare:

• infinitive: aussortieren
• main clause: Wir sortieren ... aus.

You would not say Wir aussortieren alte Kleidung in a normal main clause.

The dictionary form here is aussortieren.

• sortieren = to sort
• aussortieren = to sort out, remove, separate out, weed out

So aus- adds the idea of taking something out from a group.

That is why the sentence uses sortieren ... aus rather than just sortieren.

Because wir means we, and the verb has to match the subject.

The verb sortieren in the present tense is:

• ich sortiere
• du sortierst
• er/sie/es sortiert
• wir sortieren
• ihr sortiert
• sie/Sie sortieren

So with wir, the correct form is sortieren.

It is in the accusative case because it is the direct object of the verb.

Ask yourself: What are we sorting out?
Answer: alte Kleidung

So alte Kleidung is the thing receiving the action, which makes it accusative.

Because Kleidung is a feminine singular noun, and there is no article before it.

So the adjective takes the ending that fits:

• feminine
• singular
• accusative
• no article

That gives alte.

So:

• alte Kleidung

This is part of German adjective endings, which depend on:

1. gender
2. number
3. case
4. whether there is an article before the noun

Because German often leaves out the article when talking about something in a general, non-specific way, especially with mass nouns or uncountable nouns.

Here, Kleidung means clothing in general, not one specific set that has already been identified.

So:

• Wir sortieren alte Kleidung aus. = general clothing / old clothes

But if you meant a specific set, you might use an article:

• Wir sortieren die alte Kleidung aus.

That would sound more like the old clothes/clothing.

It is grammatically singular, even though in English clothing can refer to many items.

So German treats Kleidung as one collective or mass noun:

• die Kleidung

That is why the adjective is singular too:

• alte Kleidung

If you wanted to talk about individual items, you might use another noun, such as:

• Kleidungsstücke = items of clothing

This is a very useful question, because separable verbs change their shape depending on the structure.

1. Present tense main clause

• Wir sortieren alte Kleidung aus.

2. With a modal verb The separable verb stays together as an infinitive:

• Wir wollen alte Kleidung aussortieren.

3. Perfect tense The past participle is usually one word:

• Wir haben alte Kleidung aussortiert.

4. Subordinate clause The verb goes to the end, and the separable verb is usually together:

• ..., weil wir alte Kleidung aussortieren.

So the split form sortieren ... aus happens especially in a normal main clause.

Yes, but the conjugated verb still has to stay in the second position in a main clause.

For example:

• Wir sortieren alte Kleidung aus.
• Alte Kleidung sortieren wir aus.
• Heute sortieren wir alte Kleidung aus.

In all of these, sortieren is still the finite verb in second position, and aus still goes to the end.

That second-position rule is one of the most important word-order rules in German.