Этот опёнок червивый.

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

So:

Этот опёнок червивый
literally looks like This honey mushroom wormy,
but it means This honey mushroom is wormy.

If you were talking about the past or future, Russian would use forms of быть:

• Этот опёнок был червивый = This mushroom was wormy
• Этот опёнок будет червивый = This mushroom will be wormy

Этот means this.

It is in the masculine singular nominative form because it describes опёнок, which is a masculine singular noun in the nominative case.

Russian this changes form depending on gender, number, and case:

• этот = this, masculine
• эта = this, feminine
• это = this, neuter
• эти = these, plural

So with опёнок you need этот, not эта or это.

Опёнок is a noun meaning a kind of mushroom, often translated as honey mushroom.

It is:

• masculine
• singular
• nominative here

Its dictionary form is опёнок.

A learner may also notice the letter ё. That matters here:

• ё is pronounced like yo
• ё is always stressed

So опёнок is pronounced roughly a-PYO-nak.

Червивый means something like:

• wormy
• worm-eaten
• full of worms
• more naturally in context, infested with larvae/maggots

With mushrooms, fruit, and similar things, Russian often uses червивый when the inside has been eaten by worms or larvae.

So the sentence means that this mushroom is not good and has worms or insect larvae in it.

Червивый is an adjective, and Russian adjectives must agree with the noun they describe.

Since опёнок is:

• masculine
• singular
• nominative

the adjective also has to be:

• masculine
• singular
• nominative

That is why you get червивый.

Compare:

• Этот опёнок червивый = This mushroom is wormy
• Эта ягода червивая = This berry is wormy
• Это яблоко червивое = This apple is wormy
• Эти грибы червивые = These mushrooms are wormy

They behave similarly, but they are not exactly the same type of word.

• этот is a demonstrative word, meaning this
• червивый is a regular descriptive adjective, meaning wormy

However, in Russian grammar, both agree with the noun in gender, number, and case. So for a learner, it is useful to notice that they both match опёнок.

The sentence is simply identifying or describing the subject:

Этот опёнок = the subject, this mushroom
червивый = what is being said about it, wormy

In basic Russian sentences like this, the subject is normally in the nominative case.

So опёнок is nominative because it is the thing being talked about.

Yes, Russian word order is more flexible than English word order.

The most neutral order here is:

Этот опёнок червивый.

But other orders are possible, depending on emphasis:

• Опёнок этот червивый. = This mushroom is wormy
• Червивый этот опёнок. = This mushroom is wormy, and the speaker strongly emphasizes wormy

English usually needs a fixed order, but Russian can move words around for focus, contrast, or style.

One clue is that its singular form takes masculine agreement:

• этот опёнок
• червивый опёнок

Also, many nouns ending in a consonant are masculine, but опёнок ends in -ок, which is also a very common masculine pattern.

The best proof is agreement in the sentence:

• not эта опёнок
• not червивая опёнок
• but этот опёнок червивый

The words would change to match the noun.

Examples:

• Эта ягода червивая. = This berry is wormy
• Это яблоко червивое. = This apple is wormy
• Эти грибы червивые. = These mushrooms are wormy

So Russian keeps agreement across the sentence:

• demonstrative word
• noun
• adjective

That is one of the main things this sentence helps you notice.

No. It can be used for other things too, especially things that can have worms or larvae inside them, such as:

• mushrooms
• apples
• berries
• plums

So it is a general adjective, not one used only with mushrooms. But mushrooms and fruit are very common contexts for it.