Ядовитый мухомор красный.

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

So Ядовитый мухомор красный is understood as The poisonous fly agaric is red, even though there is no separate word for is.

This is very normal in Russian:

• Дом большой = The house is big
• Гриб ядовитый = The mushroom is poisonous

In other tenses, a form of быть appears:

• Ядовитый мухомор был красным = The poisonous fly agaric was red
• Ядовитый мухомор будет красным = The poisonous fly agaric will be red

Because they are doing two different jobs.

• ядовитый мухомор is one noun phrase: a poisonous fly agaric
• красный is the predicate adjective: it tells you something about the subject, meaning is red

So the structure is basically:

• ядовитый мухомор = subject
• красный = what is being said about the subject

In English, we would usually show that with is:

• The poisonous fly agaric is red

Russian often shows the same difference just through position and structure.

It is nominative singular.

Here, мухомор is the subject of the sentence, so nominative is the expected case. Both adjectives also match it in nominative singular masculine:

• ядовитый
• красный

So all three words are masculine singular nominative in this sentence.

A noun ending in a consonant is very often masculine in Russian, and мухомор ends in -р.

That is why the adjectives agreeing with it also take masculine forms:

• ядовитый
• красный

If the noun were feminine, the adjectives would look different:

• ядовитая
• красная

If it were neuter:

• ядовитое
• красное

Because adjectives in Russian must agree with the noun they describe or relate to in:

• gender
• number
• case

Here the noun is masculine singular nominative, so the adjectives appear in masculine singular nominative too:

• ядовитый мухомор
• мухомор красный

The exact ending can vary a little depending on spelling patterns, but -ый is a common masculine nominative singular adjective ending.

Yes. It is still an adjective.

In this sentence, it is a predicate adjective, not an adjective directly inside the noun phrase.

Compare:

• красный мухомор = a red fly agaric
• мухомор красный = the fly agaric is red

So the word class does not change. It is still an adjective; it is just being used in a different way.

Usually no, not in normal modern speech.

Russian does have short-form adjectives, and they are often used predicatively, but not every adjective commonly uses a short form today. With color adjectives, the long form is normally what you use:

• мухомор красный

The form красен does exist, but it sounds literary, elevated, or old-fashioned in most contexts.

So in ordinary modern Russian, красный is the natural choice here.

Yes, Russian word order is flexible, but changing it changes the emphasis.

This version is fairly neutral if the point is the color of the mushroom:

• Ядовитый мухомор красный

Other orders are possible, but they highlight different information. For example:

• Красный мухомор ядовитый puts more focus on poisonous
• Мухомор красный simply says The fly agaric is red

So Russian word order is not random. It often reflects what is already known and what is being emphasized.

Russian has no articles.

So ядовитый мухомор can mean:

• a poisonous fly agaric
• the poisonous fly agaric

Which one is meant depends on context, not on a separate word like a or the.

That is one reason why Russian sentences often need to be understood in context rather than word-for-word.

Yes, in some contexts it could.

In Russian, noun + adjective after the noun can sometimes appear in names, labels, or classifications, and мухомор красный is also a known species name pattern.

So without context, a learner might wonder whether красный means:

• is red
• or part of a fixed name

If your translation or lesson gives the meaning The poisonous fly agaric is red, then here красный should be understood as a predicate adjective.

So this is a good example of why context matters in Russian.