Er betrügt seinen Freund.

Because Freund is the direct object here, so it is in the accusative case.

• der Freund = nominative
• den Freund = accusative

The possessive sein- has to match that case, so:

• sein Freund = nominative
• seinen Freund = accusative

So in this sentence, seinen Freund is correct because he is the person being cheated/deceived.

It is accusative.

The verb betrügen normally takes a direct object, and that object goes in the accusative:

• Er betrügt seinen Freund.

Here:

• Er = subject, nominative
• seinen Freund = direct object, accusative

Because the verb has to be conjugated to match the subject.

The infinitive is betrügen = to cheat / to deceive.

With er (he), the present tense form is:

• ich betrüge
• du betrügst
• er/sie/es betrügt
• wir betrügen
• ihr betrügt
• sie/Sie betrügen

So Er betrügt means he cheats / he is cheating / he deceives.

Because betrügen is one of those verbs whose stem vowel changes in some present-tense forms.

The infinitive has ü already, and the singular forms keep that changed stem:

• ich betrüge
• du betrügst
• er betrügt

So this is just part of how the verb is formed. It is something you need to learn with the verb.

It can mean both, depending on context.

So Er betrügt seinen Freund could mean:

• He is deceiving his friend
• He is cheating on his boyfriend

German Freund can mean friend or boyfriend, so the sentence can be ambiguous without more context.

Grammatically, Freund is a masculine noun, but in meaning it can be:

• male friend
• boyfriend

If you want to make it clearly romantic, context usually does that. In some situations, German speakers may also say things like mein fester Freund for my boyfriend, but often just Freund is enough.

Because all nouns are capitalized in German.

So:

• Freund = noun, capitalized
• seinen = possessive determiner, not capitalized
• betrügt = verb, not capitalized

This is a basic rule of German spelling.

No, it is not separable.

Some German verbs split apart in a sentence, such as anrufen → Er ruft an.

But betrügen stays together:

• Er betrügt seinen Freund.

You do not split off be-.

Because in a normal German main clause, the conjugated verb goes in the second position.

So the pattern here is:

• Er = first element
• betrügt = verb in second position
• seinen Freund = rest of the sentence

This is the standard word order for a simple statement.

Yes, that is grammatically correct.

German allows different word orders more easily than English because case endings show who is doing what.

• Er betrügt seinen Freund. = neutral word order
• Seinen Freund betrügt er. = puts emphasis on his friend

Both mean the same basic thing, but the second version sounds more marked or emphatic.

A simple approximation is:

• beh-TROOGT

A few notes:

• be- is usually unstressed
• the stress is on trügt
• ü is a front rounded vowel, which English does not really have

A useful trick for ü:

1. Say ee as in see
2. Keep your tongue there
3. Round your lips as if saying oo

That gets you closer to German ü.

Usually yes: in this sentence, seinen naturally means his and refers to er.

So the normal reading is:

• He cheats/deceives his friend

In a larger context, seinen could sometimes refer back to another masculine person already mentioned, but without extra context, most learners should understand it as referring to the subject er.

The base possessive is sein = his / its.

It changes its ending depending on:

• case
• gender
• number of the noun that follows

Here the noun is:

• Freund = masculine singular
• in the accusative

So sein becomes seinen.

This is the same pattern you see in:

• sein Bruder = his brother
• seinen Bruder = his brother (as direct object)