Mein Bruder prüft meine Rechtschreibung, weil ich oft einen Buchstaben vergesse.

Because the possessive word mein- changes its ending depending on the gender, case, and number of the noun that follows it.

• Bruder is masculine singular and here it is the subject, so you get mein Bruder.
• Rechtschreibung is feminine singular and here it is the direct object, so you get meine Rechtschreibung.

So the difference is not about my changing meaning; it is about German grammar agreement.

This is a very common German word-order pattern.

In a main clause, the conjugated verb usually comes in position 2:

• Mein Bruder prüft meine Rechtschreibung.

But after weil (because), you have a subordinate clause, and in subordinate clauses the conjugated verb usually goes to the end:

• weil ich oft einen Buchstaben vergesse

So:

• main clause → verb in second position
• subordinate clause with weil → verb at the end

Because weil is a subordinating conjunction. Words like weil, dass, wenn, and obwohl introduce subordinate clauses, and those clauses normally send the conjugated verb to the end.

Compare:

• Ich vergesse oft einen Buchstaben.
• ..., weil ich oft einen Buchstaben vergesse.

The meaning stays the same, but the structure changes because of weil.

Because Buchstaben here is the direct object of vergesse (I forget a letter), so it is in the accusative case.

Buchstabe is a masculine noun, so the indefinite article changes like this:

• nominative: ein Buchstabe
• accusative: einen Buchstaben

That is why you see:

• ich vergesse einen Buchstaben

Also notice that the noun itself often gets an extra -n in certain forms of masculine nouns like Buchstabe.

Those are the conjugated forms for different subjects.

• mein Bruder = he, so the verb is prüft
• ich = I, so the verb is vergesse

From the infinitives:

• prüfen → er prüft
• vergessen → ich vergesse

So the endings reflect the subject:

• ich vergesse
• du vergisst
• er/sie/es vergisst

and

• ich prüfe
• du prüfst
• er/sie/es prüft

In German, adverbs like oft often come before the object, especially in a neutral sentence.

So:

• ich oft einen Buchstaben vergesse

sounds natural and straightforward.

You may also hear other word orders in different contexts, but this one is very standard. German word order is flexible in some ways, but not random.

German often combines nouns into compound nouns.

Here:

• Recht = right/correct
• Schreibung = writing/spelling

Together, Rechtschreibung means spelling/orthography.

This is completely normal in German. Instead of writing several separate words, German often builds one longer noun.

Because all nouns are capitalized in German.

So in this sentence:

• Bruder
• Rechtschreibung
• Buchstaben

are capitalized simply because they are nouns.

This is one of the most noticeable spelling rules in German.

Yes, prüft works well here and means something like checks or examines.

In everyday German, you might also hear:

• Mein Bruder kontrolliert meine Rechtschreibung.

That also means My brother checks my spelling, though prüft is perfectly natural. Different verbs can sound a bit more formal or casual depending on context.

Good observation. Buchstabe belongs to a group often called weak masculine nouns. These nouns usually add -n or -en in most forms except the nominative singular.

So:

• nominative singular: der Buchstabe
• accusative singular: den/einen Buchstaben
• dative singular: dem/einem Buchstaben

That is why the sentence has einen Buchstaben, not einen Buchstabe.

Yes:

• Weil ich oft einen Buchstaben vergesse, prüft mein Bruder meine Rechtschreibung.

That is also correct.

When the weil clause comes first, the main clause still follows the normal rule that the conjugated verb comes early, but now the whole subordinate clause counts as the first element, so the verb comes immediately after it:

• Weil ich oft einen Buchstaben vergesse, prüft ...

That is why you get prüft mein Bruder, not mein Bruder prüft in that version.