Sein Verhalten ist merkwürdig.

Here, Sein means his.

A native English speaker may confuse it with the infinitive sein = to be, but in this sentence it is a possessive determiner: his behavior.

So the sentence breaks down like this:

• Sein Verhalten = his behavior
• ist = is
• merkwürdig = strange / odd

Because it is at the beginning of the sentence, sein is capitalized as Sein. That does not mean it is a noun here.

Because Verhalten is a noun, and in German all nouns are capitalized.

• das Verhalten = the behavior
• sein Verhalten = his behavior

This is one of the most noticeable differences from English, where common nouns are usually not capitalized.

Because sein already functions like a determiner, similar to his in English.

Compare:

• das Verhalten = the behavior
• sein Verhalten = his behavior

In German, possessive words like mein, dein, sein, ihr, unser usually take the place of an article rather than appearing next to one.

So you say:

• sein Verhalten

not normally:

• das sein Verhalten

It is in the nominative case, because Sein Verhalten is the subject of the sentence.

The full subject is:

• Sein Verhalten = his behavior

The noun Verhalten is neuter: das Verhalten.
With a neuter singular noun in the nominative, the possessive determiner is:

• sein Verhalten

This is the same form you would also see with masculine nouns in the nominative, so the form alone does not tell you the gender here; you learn the gender from the noun itself: das Verhalten.

The structure is:

• Sein Verhalten = subject
• ist = verb
• merkwürdig = predicate adjective

So this is a very standard German pattern:

Subject + verb + adjective

German main clauses require the finite verb to be in the second position, and that is exactly what happens here:

• Sein Verhalten | ist | merkwürdig

Because it comes after the verb ist and works as a predicate adjective.

In German, adjectives used this way do not take adjective endings:

• Das Verhalten ist merkwürdig.
• Sein Verhalten ist merkwürdig.

But if the adjective comes before a noun, it does take an ending:

• ein merkwürdiges Verhalten = a strange behavior
• sein merkwürdiges Verhalten = his strange behavior

So:

• after sein / ist → no ending
• before a noun → adjective ending required

Merkwürdig usually means strange, odd, or peculiar.

Depending on context, it can sound:

• mildly unusual
• noticeably odd
• sometimes suspicious or concerning

Historically, the word literally suggests something like worthy of notice, but in everyday modern German it very often means strange.

So Sein Verhalten ist merkwürdig would usually be understood as:

• His behavior is strange.
• His behavior is odd.

Yes. Verhalten can often be translated as:

• behavior
• conduct
• sometimes manner

The best English word depends on context.

For example:

• Sein Verhalten ist merkwürdig.
  = His behavior is strange.

But in a more formal or moral context, conduct might sound more natural in English.

Yes. It is related to the verb sich verhalten, which means to behave.

So you can compare:

• Er verhält sich merkwürdig. = He is behaving strangely.
• Sein Verhalten ist merkwürdig. = His behavior is strange.

The first version uses a verb.
The second version uses a noun.

German often allows both kinds of expression, just like English:

• He behaves strangely.
• His behavior is strange.

Normally it refers to a person, because Verhalten usually means the way someone behaves.

So sein here most naturally means:

• his

In theory, German sein can also mean its for a neuter noun, but with Verhalten, the usual interpretation is a male person’s behavior unless the context clearly suggests something else.

A simple pronunciation guide is:

zine fer-HAL-ten ist MERK-vür-dikh

A few useful notes:

• Sein sounds roughly like English zine
• V in Verhalten is pronounced like f
• w in merkwürdig is pronounced like English v
• the ü sound in merkwürdig does not exist in normal English, so learners often need practice with it
• the final -ig is often pronounced something like -ikh in standard German

Yes, but it means something slightly different.

• Er ist merkwürdig. = He is strange.
• Sein Verhalten ist merkwürdig. = His behavior is strange.

The second sentence is a bit more specific and often a bit softer. It comments on what he is doing, not necessarily on his whole personality.

That distinction can matter:

• Er ist merkwürdig may sound like He is a weird person
• Sein Verhalten ist merkwürdig may sound more like Something about the way he is acting is odd

Yes. Then the structure changes:

• Sein Verhalten ist merkwürdig. = His behavior is strange.
• Sein merkwürdiges Verhalten ... = his strange behavior ...

When the adjective comes before the noun, it needs an ending:

• merkwürdig → merkwürdiges

That is because Verhalten is neuter singular here.

So both are correct, but they are used differently:

• predicate adjective: ist merkwürdig
• attributive adjective: merkwürdiges Verhalten