Ножка гриба находится под шляпкой.

Because Russian often expresses X of Y by putting Y in the genitive case.

So:

• ножка = stem, stalk, little leg
• гриб = mushroom
• ножка гриба = the stem of the mushroom

The dictionary form is гриб, and the genitive singular form is гриба.

This is a very common pattern in Russian:

• крыша дома = the roof of the house
• дверь комнаты = the door of the room
• ножка гриба = the stem of the mushroom

Because the preposition под changes the case of the noun that follows it.

Here it describes location: under the cap.
When под means under in the sense of location, it usually takes the instrumental case.

So:

• dictionary form: шляпка
• instrumental singular: шляпкой

That is why the sentence has под шляпкой.

A useful contrast:

• под шляпкой = under the cap, located there
• под шляпку = to under the cap, moving there

So Russian distinguishes:

• where something is → often instrumental after под
• where something moves to → often accusative after под

Находится means is located, is situated, or simply is in a location.

So in this sentence it tells you where the mushroom stem is located.

It is more explicit than just leaving the location phrase by itself. In English, is located can sound a bit formal, but in Russian находится is very normal in this kind of sentence.

Grammatically, it is the 3rd person singular present form of находиться.

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

So Russian often does not need a separate word for is:

• Дом большой. = The house is big.
• Книга на столе. = The book is on the table.

But Russian can use other verbs when it wants a more specific meaning.
Here находится does the job of is located.

So this sentence is not using a present-tense form of быть. Instead, it uses a full lexical verb meaning to be located.

Yes, that is possible.

Ножка гриба под шляпкой is shorter and more conversational. It simply states the relationship: The stem is under the cap.

Ножка гриба находится под шляпкой sounds a bit more explicit, and sometimes slightly more descriptive or textbook-like.

Both are grammatical. The version with находится just spells out the idea of location more clearly.

In Russian, ножка literally means little leg, but it is also the normal word for the stem or stalk of certain objects, including a mushroom.

So for mushroom anatomy, Russian commonly uses:

• шляпка = cap
• ножка = stem/stalk

This is idiomatic vocabulary. Even though ножка is related to нога (leg), in this context it does not mean an actual animal or human leg.

Yes. It comes from шляпа (hat) and is a diminutive form.

In mushroom vocabulary, шляпка is the standard word for the cap of a mushroom. Russian uses this image because the top of a mushroom looks like a small hat.

So although it literally relates to hat, here it is just the normal biological/everyday word for the mushroom cap.

No, Russian word order is fairly flexible because the case endings show the grammatical relationships.

The given sentence is neutral and natural:

Ножка гриба находится под шляпкой.

But other orders are also possible, for example:

• Под шляпкой находится ножка гриба.
• Ножка под шляпкой находится. — possible, but less neutral

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

A common pattern is:

• start with what you want to talk about first
• place newer or emphasized information later

The stress is:

• но́жка
• гриба́
• нахо́дится
• шля́пкой

So the full sentence is:

Но́жка гриба́ нахо́дится под шля́пкой.

A few pronunciation notes:

• ж in но́жка sounds like the s in measure
• unstressed vowels are reduced in normal speech
• тся in находится is pronounced roughly like ца

So находится sounds approximately like na-KHO-di-tsa, not exactly as it is spelled letter by letter.

Because Russian has no articles.

Russian does not have separate words corresponding directly to a/an and the. Whether you translate it as the mushroom, a mushroom, the cap, or the stem depends on context.

So:

Ножка гриба находится под шляпкой.

could be understood as something like:

• The stem of the mushroom is under the cap
• or, in a more general explanatory context, A mushroom’s stem is under the cap

The exact English article choice comes from context, not from a Russian word.