Этот белый гриб червивый.

Because гриб is a masculine singular noun. In Russian, demonstratives like this must agree with the noun in gender, number, and case.

• masculine singular: этот
• feminine singular: эта
• neuter singular: это
• plural: эти

So:

• этот гриб = this mushroom

They are both masculine singular nominative adjective forms, agreeing with гриб.

• белый гриб = white mushroom / porcini mushroom
• гриб червивый = the mushroom is wormy

Even though белый comes before the noun and червивый comes after it, both still match гриб in gender, number, and case.

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

So:

• Этот белый гриб червивый. literally: This porcini mushroom wormy. natural English: This porcini mushroom is wormy.

Russian normally leaves out есть in sentences like this.
In past or future, a form of быть does appear:

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

It can mean both literally and as a set expression, but very often белый гриб refers to the specific mushroom known in English as porcini or cep.

So in many contexts:

• белый гриб = porcini

If you treat it simply word-for-word, it is white mushroom, but learners should know that this phrase often functions like the name of a species.

Червивый means worm-eaten, wormy, or infested with larvae. It is commonly used for mushrooms, fruit, and similar foods.

It comes from червь = worm.

So a червивый гриб is a mushroom that has worms/larvae in it and is not clean inside.

The sentence is in the nominative.

• Этот белый гриб is the subject, so it is nominative.
• червивый is a predicate adjective describing the subject, and here it also appears in the nominative form.

So this is a straightforward dictionary-form style sentence.

Russian has no articles.

English distinguishes:

• a mushroom
• the mushroom
• this mushroom

Russian uses other words or context instead. Here, этот already gives a specific meaning:

• этот гриб = this mushroom

Without этот, Белый гриб червивый would sound more like The porcini mushroom is wormy / A porcini mushroom is wormy, depending on context.

Yes, but it means something different.

• Этот белый гриб червивый usually means This porcini mushroom is wormy.
• Этот гриб белый и червивый means This mushroom is white and wormy.

In the first sentence, белый гриб is likely the name of the mushroom type.
In the second, белый is clearly just an ordinary color adjective.

That is an important distinction.

Yes, Russian word order is fairly flexible, but the neutral order here is very natural:

• Этот белый гриб червивый.

You can change the order for emphasis, but the meaning or focus may shift slightly. For example:

• Червивый этот белый гриб. = That porcini mushroom is wormy. / Wormy, that porcini is.
• Этот гриб червивый. = This mushroom is wormy.

For learners, the given order is the safest and most neutral.

The stress is:

• э́тот
• бе́лый
• гриб
• черви́вый

A rough pronunciation guide:

• э́тот = EH-tuht
• бе́лый = BYE-liy
• гриб = greep
• черви́вый = chir-VEE-viy

A few useful notes:

• Final б in гриб is pronounced like п at the end: грип.
• Unstressed vowels are reduced, so the spoken form may sound less clear than the spelling suggests.

Russian does have short adjective forms, but in this sentence червивый is perfectly normal and natural.

For many adjectives, both long and short forms can appear in predicate position, but not every adjective uses the short form equally often in modern everyday speech. Here, learners should simply treat червивый as the standard, safe form to use.

So:

• Этот белый гриб червивый. = completely normal