В лукошке лежат подберёзовики и подосиновики.

Because after в meaning in, Russian uses the prepositional case to show location.

• лукошко = nominative singular
• в лукошке = in the basket

So this sentence uses:

• в
  • prepositional = location: in the basket
• compare with в лукошко = into the basket (motion toward)

This is a very common contrast:

• в доме = in the house
• в дом = into the house

Because the subject is plural: подберёзовики и подосиновики.

Both nouns are plural, and they are joined by и (and), so the verb must also be plural:

• лежит = singular
• лежат = plural

Compare:

• В лукошке лежит подберёзовик. = A birch bolete is lying in the basket.
• В лукошке лежат подберёзовики. = Birch boletes are lying in the basket.
• В лукошке лежат подберёзовики и подосиновики. = Birch boletes and aspen boletes are lying in the basket.

Russian often prefers a more specific verb where English might simply use are.

лежать / лежат means to lie or to be lying. It describes the physical position of the objects. Mushrooms in a basket are thought of as lying there.

Russian often uses different verbs depending on position:

• лежать = lie
• стоять = stand
• сидеть = sit
• висеть = hang

So В лукошке лежат... sounds natural because the mushrooms are resting in the basket, not standing upright.

They are in the nominative plural, because they are the subject of the sentence.

Singular and plural:

• подберёзовик → подберёзовики
• подосиновик → подосиновики

Since they are the things doing the grammatical job of the subject, they stay in the nominative.

The dictionary forms are the singular nominative forms:

• подберёзовик
• подосиновик

In the sentence, you see their plural forms:

• подберёзовики
• подосиновики

A learner often wants to know this because Russian dictionaries normally list nouns in the singular nominative.

Yes. Лукошко is a neuter noun ending in -о.

Its forms include:

• nominative singular: лукошко
• prepositional singular: в лукошке

That -о → -е change is normal for many neuter nouns in the prepositional singular after в, на, о, etc.

Compare:

• окно → в окне
• море → на море
• лукошко → в лукошке

Also, лукошко has a somewhat rustic or traditional feel; it means a small basket, often the kind used for gathering berries or mushrooms.

Both are possible. Russian word order is flexible.

The version you have:

• В лукошке лежат подберёзовики и подосиновики.

starts with the location, so it feels like:

• In the basket, there are ...
• or In the basket lie ...

If you say:

• Подберёзовики и подосиновики лежат в лукошке.

the focus is more on the mushrooms themselves.

So the choice is often about what information comes first and what the speaker wants to emphasize, not about strict grammar.

Because Russian has no articles.

English needs words like a, an, and the, but Russian usually does not. Whether the meaning is definite or indefinite is understood from context.

So:

• В лукошке лежат подберёзовики и подосиновики.

can correspond to English with the, some, or sometimes no article-like emphasis at all, depending on context.

These words are formed historically from the idea of growing under or near certain trees:

• подберёзовик is connected with берёза = birch
• подосиновик is connected with осина = aspen

So the forms are built from meaningful parts, even though for a learner it is best to treat them first as vocabulary items.

This kind of word formation is common in Russian and can help with memory once you notice the tree names inside them.

In printed Russian, ё is often replaced by е, so you may also see:

• подберезовики

But the pronunciation and stress are normally the same as if ё were written. In careful teaching materials, dictionaries, or learner texts, ё is often kept because it helps with pronunciation.

So:

• подберёзовики = clearer for learners
• подберезовики = also common in real-life writing

The main word stresses are:

• в лукОшке
• лежАт
• подберЁзовики
• подосИновики

Stress matters in Russian and is not always predictable, so it is worth learning each word together with its stress.

You could, but it would sound different.

• лежат describes the physical position: the mushrooms are lying in the basket.
• находятся means something like are located / are situated and sounds more formal or neutral.

So in everyday Russian, лежат is the natural choice here. Находятся would usually sound less vivid and less idiomatic for mushrooms in a basket.

They are inanimate nouns, and in this sentence it does not create any special complication because they are in the nominative plural.

Animacy matters most when you use forms like the accusative plural, where animate and inanimate nouns behave differently. But here, as the subject, they are simply nominative plural:

• подберёзовики
• подосиновики

So for this sentence, you do not need to worry much about animacy beyond knowing they are grammatically inanimate.