На диване лежала чистая пелёнка, а рядом — яркая погремушка.

Because лежала agrees with пелёнка, which is a feminine singular noun.

In the past tense, Russian verbs change for:

• gender in the singular
• number

So:

• лежал = masculine
• лежала = feminine
• лежало = neuter
• лежали = plural

Since пелёнка is feminine, Russian uses лежала.

They are adjectives agreeing with feminine singular nouns in the nominative case:

• чистая пелёнка
• яркая погремушка

Russian adjectives must match the noun in:

• gender
• number
• case

Here both nouns are feminine singular and are the main nouns being described, so the adjectives take the feminine nominative singular ending -ая.

Because this sentence describes location, not movement.

With на, Russian often uses:

• accusative for motion onto something
• prepositional for being on something

So:

• на диван = onto the sofa
• на диване = on the sofa

Here the cloth is already there, so на диване is correct.

Russian often prefers a position verb when describing where something is.

So instead of just saying there was a clean diaper on the sofa, Russian naturally says something like:

• На диване лежала чистая пелёнка
  literally: A clean cloth/diaper was lying on the sofa

This sounds normal in Russian, especially for objects resting on a surface.

Common verbs like this are:

• лежать = to lie
• стоять = to stand
• висеть = to hang

Russian often chooses the verb based on the object's physical position.

Пелёнка usually means a baby cloth, swaddling cloth, or cloth diaper/changing cloth, depending on context.

It is related to baby care and wrapping. In modern everyday speech, if you specifically mean a disposable diaper, Russian more often uses:

• подгузник

So пелёнка is not always exactly the same as English diaper. It can be a cloth used for swaddling, lining, or changing a baby.

Погремушка means rattle, usually a baby’s toy rattle.

It is feminine because nouns ending in -а or -я are very often feminine in Russian.

So:

• погремушка = feminine
• therefore яркая погремушка
• and if a past-tense verb were repeated, it would also be feminine: лежала

The dash shows that something has been left out, usually because it is easy to understand from the first part.

The full idea is something like:

• На диване лежала чистая пелёнка, а рядом лежала яркая погремушка.

But Russian often omits the repeated verb in the second clause:

• а рядом — яркая погремушка

This is very natural and makes the sentence more compact and elegant.

In this kind of sentence, а connects two parts of a description and often gives a slight sense of contrast, separate placement, or meanwhile/as for the other thing.

So here:

• На диване лежала чистая пелёнка, а рядом — яркая погремушка.

This feels like:

• On the sofa there was a clean cloth, and beside it, a bright rattle.

If you used и, it would sound more like simple addition. А is very common when listing parts of a scene from different angles or locations.

Here рядом is an adverb meaning nearby, next to it, or beside it.

In this sentence it stands on its own:

• а рядом — яркая погремушка
• and nearby / next to it — a bright rattle

It does not need a noun after it here because the reference is clear from context: next to the sofa/next to the cloth/in that nearby place.

You may also see:

• рядом с диваном = next to the sofa
• рядом с ней = next to her

But in this sentence, just рядом is enough.

Russian word order is flexible, and the first position often sets the scene or tells you where to look first.

So:

• На диване лежала чистая пелёнка
  focuses first on the location: On the sofa...
• Чистая пелёнка лежала на диване
  starts with the object itself: A clean cloth was lying on the sofa

Both are grammatical, but the original sounds more like a visual description of a scene.

Russian has no articles.

So Russian does not have direct equivalents of English:

• a
• an
• the

Whether something is understood as a clean cloth or the clean cloth depends on context.

In this sentence, English usually uses a because the objects are being introduced for the first time:

• a clean cloth
• a bright rattle

Russian leaves that information to context rather than grammar.