Подосиновик и белый гриб лежат рядом.

Because the subject is compound: подосиновик и белый гриб = an aspen bolete and a porcini mushroom.

In Russian, when two nouns are joined by и (and), the verb is normally plural:

• гриб лежит = the mushroom is lying
• подосиновик и белый гриб лежат = the aspen bolete and the porcini mushroom are lying

So лежат is the 3rd person plural form of лежать.

Лежать means to lie, to be lying, or more generally to be placed somewhere in a lying position.

In this sentence, лежат рядом means something like:

• are lying next to each other
• are lying nearby
• are side by side

Russian often uses лежать for objects that are physically resting somewhere, especially on a surface.

Рядом means near, nearby, or next to.

It can be used in two common ways:

1. On its own, as in this sentence:
   лежат рядом = are lying nearby / next to each other

2. With с

   • instrumental case:
     рядом с домом = next to the house

So in your sentence, рядом stands by itself and means that the two mushrooms are close together.

Grammatically, yes: белый = white, гриб = mushroom.

But in Russian, белый гриб is also the normal name for a specific mushroom: porcini or cep.

So this phrase is both:

• literally: white mushroom
• idiomatically: porcini mushroom

This is very common in Russian mushroom vocabulary: ordinary-looking adjective + noun combinations can function as standard species names.

Because гриб is a masculine noun.

Russian adjectives must agree with the noun in:

• gender
• number
• case

Here:

• гриб is masculine
• singular
• nominative case

So the adjective must also be masculine singular nominative:

• белый гриб

Compare:

• белая книга = white book, because книга is feminine
• белое вино = white wine, because вино is neuter

Подосиновик is the name of a mushroom species, usually called aspen bolete in English.

Literally, the word is related to осина = aspen tree. The prefix под- can suggest under or near, so the word has the sense of a mushroom associated with aspens.

A learner does not need to analyze it every time, though. It is best learned as one vocabulary item:

• подосиновик = aspen bolete

No, Russian word order is fairly flexible.

Подосиновик и белый гриб лежат рядом is a neutral, natural order.

You could also say:

• Рядом лежат подосиновик и белый гриб
• Подосиновик и белый гриб рядом лежат

These versions shift the focus or emphasis, but the basic meaning stays the same.

In English, word order is more rigid. In Russian, endings and verb forms often carry the grammar, so word order can be used more for style and emphasis.

Because Russian has no articles.

So подосиновик can mean:

• an aspen bolete
• the aspen bolete

and белый гриб can mean:

• a porcini mushroom
• the porcini mushroom

The exact meaning depends on context. Russian speakers understand definiteness from the situation, previous mention, or emphasis, not from separate words like a or the.

Yes, that is a good natural translation.

Depending on context, you could also say:

• The aspen bolete and the porcini mushroom are lying next to each other
• The aspen bolete and the porcini are side by side
• An aspen bolete and a porcini mushroom are lying nearby

Which English version is best depends on whether the context is definite or indefinite and whether you want to keep the physical sense of лежат.

The main stresses are:

• подосИновик
• бЕлый
• гриб
• лежАт
• рЯдом

So the sentence is pronounced roughly as:

подосИновик и бЕлый гриб лежАт рЯдом

Stress is important in Russian, because it is not always predictable and can affect how natural your speech sounds.