Это странное поведение.

In Russian, the verb to be is usually omitted in the present tense.

So where English says:

• This is strange behavior

Russian simply says:

• Это странное поведение

Literally, it is more like:

• This strange behavior

but the meaning is understood as This is strange behavior.

In the past or future, Russian does use forms of to be:

• Это было странное поведение. — That was strange behavior.
• Это будет странное поведение. — That will be strange behavior.

Here это most naturally means this or that, depending on context. It often points to a situation, action, or thing that has just been mentioned or is being observed.

So Это странное поведение can mean:

• This is strange behavior
• That is strange behavior

Sometimes in English you might also translate it as It’s strange behavior, especially if это refers to something obvious from context.

A useful idea: это often works as a general pointing word, not always a neat one-to-one match with English this.

Because странное must agree with the noun поведение.

The noun поведение is:

• singular
• neuter
• nominative here

So the adjective must also be:

• singular
• neuter
• nominative

That gives:

• странное поведение — strange behavior

Compare:

• странный человек — a strange man/person
• странная идея — a strange idea
• странное поведение — strange behavior

A very common clue is the ending. Many Russian nouns ending in -е or -о are neuter.

So:

• поведение ends in -ие / -е
• therefore it is neuter

Because it is neuter, adjectives modifying it take neuter forms:

• странное поведение
• хорошее поведение
• плохое поведение

As always, there are some exceptions in Russian, but this pattern is very common and reliable.

It is in the nominative case.

In this sentence, поведение is the main noun in the predicate phrase:

• Это странное поведение.

The phrase странное поведение is in the nominative because it names or identifies what это is.

You can think of it as similar to:

• Это книга. — This is a book.
• Это проблема. — This is a problem.
• Это странное поведение. — This is strange behavior.

Yes, you can, but the emphasis changes.

• Это странное поведение. — This is strange behavior.
• Поведение странное. — The behavior is strange.

The second version sounds more like you are talking specifically about the behavior and commenting on it.

Russian word order is flexible, so different orders can sound more natural in different contexts. But the original sentence with это is especially good when you are pointing something out or identifying it.

Yes, it is possible, but it sounds more marked and usually puts extra emphasis on странное.

Compare:

• Это странное поведение. — the most neutral wording
• Это поведение странное. — This behavior is strange; more contrastive or emphatic

The second version might be used if you are comparing behaviors:

• Это поведение странное, а то — обычное.
• This behavior is strange, but that one is normal.

So yes, it is grammatical, but it is not the most neutral default sentence.

Yes. Странное поведение is a noun phrase made up of:

• странное — adjective, strange
• поведение — noun, behavior

Together they mean:

• strange behavior

Then это points to that whole idea:

• Это [странное поведение].

So structurally, the sentence is basically:

• This/that = strange behavior

A natural pronunciation is:

• Э́то стра́нное поведе́ние.

Approximate stress:

• Э́-то
• СТРА-нно-е
• по-ве-ДЕ-ни-е

A rough English-style approximation would be:

• EH-ta STRAHN-na-ye pa-vye-DYE-nee-ye

A few notes:

• это has stress on the first syllable
• странное has stress on стра́-
• поведение has stress on -де́-

Also, spoken Russian often smooths sounds together a bit, so странное may sound less separated than its spelling suggests.

Both are normal neuter adjective endings in Russian, depending on the adjective stem.

Examples:

• странное поведение
• новое место
• синее море
• хорошее решение

So the neuter nominative singular ending may appear as:

• -ое
• -ее

Which one you get depends on the adjective itself, not on the noun.

For странный, the neuter form is:

• странное

No. Это does not change here to match the gender of поведение.

That is why you say:

• Это странный человек.
• Это странная идея.
• Это странное поведение.

In all three examples, это stays the same.

The adjective changes to match the noun, but это does not.

Usually not in this sentence.

• это is a pointing word: this / that / it
• оно means it, but as a regular third-person pronoun referring back to a neuter noun

So:

• Это странное поведение. — This/that is strange behavior.

But оно would normally need a clear neuter noun already mentioned:

• Поведение было необычным. Оно всех удивило.
• The behavior was unusual. It surprised everyone.

So in your sentence, это is the natural choice.

Russian has no articles like a or the.

So странное поведение could be understood in English as:

• strange behavior
• a strange behavior in some contexts, though this is less natural in English
• the strange behavior, if context makes it specific

Usually in English, behavior is often treated as an uncountable noun, so the best translation is often just:

• strange behavior

Russian does not mark this with an article, so you rely on context.

They are related, but not the same.

• Это странное поведение. — This is strange behavior.
  • You are naming what it is: behavior
• Это странно. — This is strange.
  • You are simply describing it as strange

So:

• Это странно = shorter, more general
• Это странное поведение = more specific, because you identify it as behavior

Both are correct; they just say slightly different things.