Это интересное выражение.

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

So:

• Это интересное выражение. = This is an interesting expression.

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

• Это было интересное выражение. = This was an interesting expression.
• Это будет интересное выражение. = This will be an interesting expression.

So the missing is is completely normal.

Here это means this or that, depending on context.

In this sentence, это points to something and introduces what it is:

• Это интересное выражение. = This is an interesting expression.

A useful thing to know is that это does not change for gender:

• Это хороший фильм. = This is a good film.
• Это хорошая книга. = This is a good book.
• Это хорошее слово. = This is a good word.

So even though выражение is neuter, это stays это.

Because it agrees with выражение, which is a neuter singular noun.

Russian adjectives must match the noun in:

• gender
• number
• case

The adjective интересный changes like this in the nominative singular:

• masculine: интересный
• feminine: интересная
• neuter: интересное

Since выражение is neuter, you need:

• интересное выражение

not:

• интересный выражение
• интересная выражение

A very common clue is the ending.

Выражение ends in -ие / -ение, and nouns of this type are usually neuter:

• выражение = expression
• предложение = sentence / proposal
• здание = building
• упражнение = exercise

Also, many neuter nouns in Russian end in:

• -о
• -е
• -ие
• -ение

So выражение is neuter singular nominative here.

Both интересное and выражение are in the nominative case.

Why? Because this is a basic identifying sentence:

• Это интересное выражение. = This is an interesting expression.

The noun phrase интересное выражение is the thing being identified, so it appears in the nominative.

You can see the adjective-noun agreement clearly:

• интересное = nominative singular neuter
• выражение = nominative singular neuter

Yes, Russian word order is flexible, but the neutral version is:

• Это интересное выражение.

Other orders are possible, but they may sound more marked or put emphasis in different places. For example:

• Интересное выражение. = An interesting expression. / It’s an interesting expression.
• Выражение интересное. = The expression is interesting.
• Это выражение интересное. = This expression is interesting.

So your original sentence is the most straightforward way to say This is an interesting expression.

The stress is:

• Э́то интере́сное выраже́ние.

Stress falls on:

• Э́то
• интере́сное
• выраже́ние

A rough English-style approximation would be:

• EH-tuh een-tee-RYES-nuh-yee vuh-rah-ZHYE-nee-yeh

But it is better to listen to native audio if possible, because Russian vowel reduction affects unstressed vowels.

Not always exactly.

Выражение often means:

• expression
• phrase
• way of saying something

Depending on context, it can refer to:

• a fixed expression
• a spoken phrase
• a figurative expression
• even a facial expression in other contexts

So in this sentence, it most naturally means expression or phrase.

Usually, Это интересное выражение is understood as:

• This is an interesting expression.

If you specifically want to say This expression is interesting, Russian more naturally says:

• Это выражение интересное.
• or more commonly: Это выражение интересно.

So the original sentence is mainly an identification sentence, not a comment about a specific expression already established as the topic.

Because интересное is an adjective describing the noun выражение.

• интересное выражение = an interesting expression

But интересно is usually a short-form predicate or an adverb-like form:

• Это интересно. = This is interesting.
• Мне интересно. = I’m interested. / It’s interesting to me.

So:

• Это интересное выражение. = This is an interesting expression.
• Это интересно. = This is interesting.

Those are related, but they are different structures.