Mein Bruder findet seinen Schlüssel nicht.

Here is the basic breakdown:

• Mein Bruder = the subject, the person doing the action
• findet = the verb, finds / is finding
• seinen Schlüssel = the direct object, the thing being looked for
• nicht = not

So the structure is:

• subject + verb + object + not

In grammatical terms:

• Mein Bruder is in the nominative
• seinen Schlüssel is in the accusative

Because the verb has to match the subject.

The infinitive is finden = to find.

With mein Bruder = my brother = he, German uses the 3rd person singular form:

• ich finde
• du findest
• er/sie/es findet
• wir finden
• ihr findet
• sie/Sie finden

So:

• Mein Bruder findet = My brother finds / is finding

Because Schlüssel is a masculine noun and it is the direct object, so it is in the accusative case.

The noun is:

• der Schlüssel = the key

Masculine words change in the accusative, and the possessive determiner changes too:

• nominative: sein Schlüssel = his key
• accusative: seinen Schlüssel = his key

So after the verb findet, you need the accusative form:

• seinen Schlüssel

The -en ending is there because of case, not because of meaning.

You learn it together with its article:

• der Schlüssel = the key

That tells you it is masculine.

This matters because masculine nouns are the ones that show a visible change in the accusative singular:

• der Schlüssel → den Schlüssel
• sein Schlüssel → seinen Schlüssel
• mein Schlüssel → meinen Schlüssel

So knowing the gender of the noun is very important in German.

Because seinen already works like a determiner.

In German, possessive words such as:

• mein = my
• dein = your
• sein = his/its
• ihr = her/their

usually take the place of an article.

So you say:

• sein Schlüssel = his key

not:

• sein der Schlüssel

Just like in English, you normally say his key, not his the key.

Because in this sentence nicht is negating the verbal idea find his key.

A very common pattern is:

• subject + verb + object + nicht

So:

• Mein Bruder findet seinen Schlüssel nicht.

This is the normal word order for My brother doesn’t find / can’t find his key.

In German, nicht often comes:

• before the word or phrase it specifically negates, or
• near the end of the clause if it negates the whole predicate

Here, putting nicht after seinen Schlüssel sounds natural and standard.

No, that is not normal German main-clause word order.

In a standard German main clause, the conjugated verb usually comes in second position:

• Mein Bruder findet seinen Schlüssel nicht.

So after Mein Bruder, the verb findet must come right away.

That is one of the most important German word-order rules: verb second in main clauses.

Usually, in a sentence like this, yes, people will normally understand it as his own key.

But grammatically, seinen can sometimes be ambiguous. It can mean:

• the subject’s key, or
• another male person’s key, depending on context

In everyday usage, with Mein Bruder as the subject, most people will naturally read seinen Schlüssel as my brother’s key.

If you wanted to make his own key extra clear, you could say:

• seinen eigenen Schlüssel = his own key

Because kein is used to negate a noun phrase without a specific possessor/article meaning, while nicht negates the sentence or verb phrase.

Here you are talking about a specific key:

• seinen Schlüssel = his key

So you say:

• Mein Bruder findet seinen Schlüssel nicht. = My brother can’t find his key.

Compare that with:

• Mein Bruder findet keinen Schlüssel. = My brother can’t find any key / doesn’t find a key.

That has a different meaning. It is less specific.

Because all German nouns are capitalized.

So:

• Bruder is a noun
• Schlüssel is a noun

That is why they begin with capital letters.

Mein is capitalized here only because it is the first word of the sentence. If it appeared in the middle, it would normally be lowercase:

• Ich sehe meinen Bruder.

Yes, German word order is flexible, but the conjugated verb still has to stay in second position in a main clause.

For example, you can also say:

• Seinen Schlüssel findet mein Bruder nicht.

This still means basically the same thing, but now seinen Schlüssel is emphasized more, roughly like:

• His key, my brother can’t find.

So the original sentence is the most neutral version, but other word orders are possible for emphasis.