Сыроежка не червивая.

In Russian, the present-tense form of to be is usually omitted.

So:

• Сыроежка червивая. = The russula is wormy.
• Сыроежка не червивая. = The russula is not wormy.

Russian does not normally say есть here. Using есть in a sentence like this would sound unnatural in standard modern Russian.

Сыроежка is a feminine singular noun. Many Russian nouns ending in -а or -я are feminine.

That matters because the adjective has to agree with it in:

• gender
• number
• case

So because сыроежка is feminine singular nominative, the adjective also appears in the feminine singular nominative form: червивая.

Russian adjectives change their endings to match the noun they describe.

Here the noun is:

• сыроежка — feminine
• singular
• nominative

So the adjective must match:

• masculine: червивый
• feminine: червивая
• neuter: червивое
• plural: червивые

That is why the sentence uses червивая.

Не negates what follows it.

So:

• червивая = wormy
• не червивая = not wormy

In this sentence, не negates the predicate adjective. It tells you that the mushroom does not have that quality.

With adjectives used as the main predicate of a sentence, Russian often writes не separately when it is simple factual negation.

So:

• Сыроежка не червивая. = The russula is not wormy.

Writing it separately is the normal choice here.

You may sometimes see adjectives with не written together in other contexts, but that usually suggests a more fixed opposite quality, or follows different spelling rules. In this sentence, separate spelling is the straightforward and natural option.

Yes. Червивый / червивая literally means worm-eaten, wormy, or full of worms.

It is commonly used for:

• mushrooms
• fruit
• sometimes other food or plants

So in this sentence, it means the mushroom is not infested with worms or not worm-eaten.

It is in the nominative singular.

This is because сыроежка is the subject of the sentence:

• Сыроежка = the russula
• не червивая = is not wormy

The adjective червивая is also in the nominative singular feminine, because it agrees with the subject in this kind of sentence.

Yes. Russian word order is fairly flexible.

The neutral order here is:

• Сыроежка не червивая.

But you could also hear:

• Не червивая сыроежка.
• Червивая сыроежка? Нет, не червивая.

Changing the order usually changes the focus or emphasis, not the basic meaning.

For a simple textbook statement, Сыроежка не червивая is the most neutral version.

A helpful approximation is:

• сыроежка → si-ro-YEZH-ka
• не → nye
• червивая → chir-VEE-va-ya

So the whole sentence is roughly:

• si-ro-YEZH-ka nye chir-VEE-va-ya

The main stresses are:

• сырое́жка
• черви́вая

It is only grammatically feminine here.

Russian nouns have grammatical gender, but that does not mean the thing itself is biologically female. A mushroom is not being described as female; the noun simply belongs to the feminine gender class, and the adjective agrees with that grammar.

Yes, depending on context.

Russian does not have articles like a or the, so Сыроежка can mean:

• a russula
• the russula
• this russula

The exact English translation depends on the situation. The Russian sentence itself does not specify that.