В классе у окна стоит чёрный рояль, а рядом лежат ноты.

Because this sentence describes location, not movement.

• в класс = into the classroom (direction, motion)
• в классе = in the classroom (location)

After в, Russian uses:

• Accusative for motion toward a place
• Prepositional for being in a place

So класс becomes в классе in the prepositional case.

У usually means by, near, or next to.

So у окна means by the window / near the window.

The preposition у requires the genitive case, so:

• окно = window
• у окна = by the window

This is a very common pattern:

• у двери = by the door
• у стены = by the wall
• у дома = near the house

Russian often uses different verbs for the way objects are positioned in space.

Here:

• стоять = to stand
• лежать = to lie

A piano is thought of as something that is upright on its legs, so Russian says it stands:

• рояль стоит

In English we usually just say there is or is standing less literally, but Russian prefers these position verbs.

Because ноты are imagined as something lying on a surface, not standing.

So Russian says:

• ноты лежат = the sheet music is lying / the notes are lying

This is the natural verb for flat objects such as:

• книга лежит на столе = the book is lying on the table
• документы лежат здесь = the documents are lying here

Since ноты is plural, the verb is also plural:

• лежат = they lie

In the present tense, Russian normally omits the verb to be.

So instead of saying something like the piano is in the classroom, Russian simply says:

• В классе ... стоит рояль

This is completely normal. Russian does use forms of быть in other tenses:

• был = was
• будет = will be

But in the present tense, is/are/am is usually not expressed.

Russian word order is more flexible than English word order. The sentence begins with the location:

• В классе у окна = In the classroom by the window

Then it introduces what is there:

• стоит чёрный рояль

This order is very natural in Russian when describing a scene. It often goes:

1. Where
2. What is there

So the sentence sounds like a description of a room: first the setting, then the objects in it.

Both words are in the nominative singular masculine because рояль is the subject of the clause.

• рояль = grand piano
• чёрный = black

The adjective must agree with the noun in:

• gender
• number
• case

So:

• рояль is masculine singular nominative
• therefore чёрный is also masculine singular nominative

Yes, рояль is masculine.

Even though it ends in -ь, that ending can be either masculine or feminine in Russian. You just have to learn the gender of such nouns individually.

Examples:

• рояль = masculine
• день = masculine
• дверь = feminine
• тетрадь = feminine

That is why the adjective is чёрный, not чёрная.

In this sentence, а links two parts of the description:

• В классе у окна стоит чёрный рояль, а рядом лежат ноты.

Here а is best understood as and, but with a slight sense of meanwhile / while / and also. It often connects two different pieces of information, sometimes with a mild contrast.

• и simply adds
• а often sets one scene element beside another

So here а sounds very natural when describing one object and then another nearby object.

Рядом means nearby, next to it, or close by.

In this sentence:

• а рядом лежат ноты = and nearby there is sheet music lying there

Very often рядом can also be used with с:

• рядом с окном = next to the window
• рядом с домом = next to the house

But here it is used on its own as an adverb: nearby.

Literally, ноты is the plural of нота, meaning note. But in many contexts, especially like this one, ноты usually means sheet music.

So in this sentence, it most likely means that some sheet music is lying near the piano, not that individual abstract musical notes are somehow lying there.

Context decides the most natural translation.

Both orders are possible in Russian, but they can sound slightly different.

• Ноты лежат рядом focuses more on the notes
• Рядом лежат ноты sounds more descriptive and scene-setting, like nearby there are notes lying

Russian often places the location or adverb first when presenting new information. So this word order is very natural in descriptions.

A helpful stress guide is:

• В кла́ссе у окна́ стои́т чёрный роя́ль, а ря́дом лежа́т но́ты.

A few useful notes:

• стоит = stah-EET
• рояль = rah-YAL'
• лежат = lee-ZHAT
• ноты = NO-ty

Stress is important in Russian, so it is worth learning words together with their stressed syllables.