Ich bitte meinen Bruder um Hilfe.

Because Bruder is the direct object of bitten, and bitten takes the accusative case for the person you ask.

• der Bruder = nominative
• meinen Bruder = accusative

So:

• Ich bitte meinen Bruder ... = I ask my brother ...

This change only shows clearly on masculine nouns. If it were a feminine noun, the article might look the same in nominative and accusative.

It is accusative singular masculine.

You can see that from:

• mein Bruder = nominative
• meinen Bruder = accusative

This is a very common pattern in German: masculine words often change in the accusative, while neuter and feminine often do not change as much.

Because the verb pattern is:

jemanden um etwas bitten = to ask someone for something

So the sentence is built like this:

• Ich = subject
• bitte = verb
• meinen Bruder = the person I am asking
• um Hilfe = the thing I am asking for

So um is not random here. It belongs to the expression jemanden um etwas bitten.

Here um takes the accusative.

That means in the structure um etwas:

• um Hilfe
• um Geld
• um Rat

In this sentence, Hilfe does not visibly change, but grammatically it is in the accusative because um requires it.

German often leaves out the article with abstract or general nouns, especially in common expressions.

So um Hilfe means for help in a general sense.

This is more natural than:

• um die Hilfe = for the specific help
• um eine Hilfe = for a help, which usually sounds unnatural here

So um Hilfe bitten is the normal idiomatic expression.

Not exactly.

• fragen usually means to ask a question or ask someone something
• bitten means to ask/request something, often with a sense of asking for help, a favor, permission, or support

So in this sentence, bitten is the better verb because the meaning is a request, not just asking a question.

Compare:

• Ich frage meinen Bruder etwas. = I ask my brother something.
• Ich bitte meinen Bruder um Hilfe. = I ask my brother for help.

Some learners try to say that, but standard German prefers bitten here.

The idiomatic form is:

jemanden um Hilfe bitten

Using fragen with um Hilfe is generally not the normal choice in standard German. If you want to express a request for help, bitten is the safer and more natural verb.

Not with bitten.

German verbs do not always work like English logic might suggest. With bitten, the person you ask is in the accusative, not the dative.

So the correct pattern is:

jemanden um etwas bitten

not

jemandem um etwas bitten

That is something you simply need to learn with the verb.

Yes. They are related.

• ich bitte = I ask / I request
• bitte! = please / here you are / you're welcome, depending on context

They come from the same verb bitten. In this sentence, though, bitte is just the 1st person singular present tense form of the verb:

• ich bitte
• du bittest
• er/sie/es bittet

Yes, bitten is a strong verb.

Important forms:

• ich bitte = I ask
• du bittest
• er/sie/es bittet

Past forms:

• ich bat = I asked
• ich habe gebeten = I have asked / asked

So the main principal parts are:

• bitten
• bat
• gebeten

Notice that in the present tense, the stem vowel changes in du and er/sie/es, but not in ich.

Yes. A common alternative is:

Ich bitte meinen Bruder, mir zu helfen.

That means something like I ask my brother to help me.

So German can express the same general idea in at least two common ways:

• jemanden um Hilfe bitten = ask someone for help
• jemanden bitten, etwas zu tun = ask someone to do something

Both are useful patterns.

The normal word order in a main clause is:

Ich bitte meinen Bruder um Hilfe.

That is the most neutral order.

You can change word order for emphasis, but the verb still stays in second position in a main clause. For example:

• Meinen Bruder bitte ich um Hilfe.

This puts emphasis on my brother. It is grammatical, but less neutral.

So for learners, the basic pattern to remember is:

Subject + verb + person (accusative) + um + thing

A very useful pattern is:

jemanden um etwas bitten

Meaning:

to ask someone for something

Examples:

• Ich bitte meinen Bruder um Hilfe.
• Sie bittet ihren Freund um Rat.
• Wir bitten den Lehrer um Geduld.

If you learn the whole pattern instead of translating word by word, it will be much easier to use correctly.