Рядом с сосной трава была выше, а на песке у озера играли дети.

Because с in the expression рядом с requires the instrumental case.

• dictionary form: сосна = pine tree
• instrumental singular: сосной

So:

• рядом с сосной = next to / beside the pine tree

This is a very common pattern:

• рядом с домом = next to the house
• рядом с рекой = next to the river

Here рядом с works as a fixed expression meaning next to / beside.

Because it is a feminine noun ending in -а, and in the instrumental singular that ending usually changes to -ой or -ей.

So:

• сосна → сосной

This is a normal pattern:

• мама → мамой
• книга → книгой
• сестра → сестрой

So сосной is just the correct instrumental singular form of сосна.

Because на here means on or on the surface of, and with location it normally takes the prepositional case.

• dictionary form: песок = sand
• prepositional singular: на песке = on the sand

So:

• на песке = on the sand

Compare:

• на столе = on the table
• на полу = on the floor
• на берегу = on the shore

Here the children are located on the sand, so Russian uses на + prepositional.

Because the preposition у requires the genitive case, not the prepositional.

• dictionary form: озеро = lake
• genitive singular: озера

So:

• у озера = by the lake / near the lake

This is a very common rule:

• у дома = by the house
• у окна = by the window
• у моря = by the sea

So у озера is correct because у always takes the genitive.

Yes, both can express nearness, but they are built differently and often feel slightly different.

• рядом с + instrumental = right next to / beside
• у + genitive = by / near / at

So:

• рядом с сосной suggests something like right beside the pine tree
• у озера means by the lake or near the lake

Very roughly, рядом с can sound a little more specifically next to, while у is often broader: by, at, near.

Because была is the past tense of быть and it agrees with трава.

• трава is feminine singular
• past tense in Russian agrees in gender and number

So:

• masculine singular: был
• feminine singular: была
• neuter singular: было
• plural: были

Since трава is feminine singular, the sentence uses была:

• трава была выше = the grass was taller/higher

Выше is the comparative form of высокий and means higher / taller.

Russian often uses short comparative forms like this:

• высокий → выше
• большой → больше
• маленький / малый → меньше
• хороший → лучше

So:

• трава была выше = the grass was taller / higher

Russian usually prefers this simple comparative form in ordinary speech.
Более высокая is possible in some contexts, but it sounds more formal or less natural here.

Because Russian, like English, can leave that comparison understood from context.

• трава была выше literally = the grass was taller/higher

This implies comparison with something unstated, such as:

• higher than elsewhere
• higher than usual
• higher than in another place already mentioned in the wider context

English does this too:

• The grass was taller near the pine tree.

You do not have to explicitly say than... if the context already makes it clear.

Because играли is past tense plural, and it agrees with дети.

• дети = children
• this noun is grammatically plural
• so the verb must also be plural in the past tense

Past tense agreement:

• играл = he played
• играла = she played
• играло = it played
• играли = they played

So:

• играли дети = the children were playing / played

Both are possible, but the word order changes the emphasis.

• дети играли = neutral, straightforward the children were playing
• играли дети = puts more focus on the action or scene first, and then introduces children as the subject

Russian word order is more flexible than English because case endings show grammatical roles. In this sentence, играли дети fits the descriptive, narrative style:

• а на песке у озера играли дети
  = and on the sand by the lake, children were playing

It sounds natural in scene-setting or storytelling.

Here а links two parts of the sentence and shows a mild contrast or a shift of attention.

The sentence has two pictures:

• near the pine tree, the grass was taller
• on the sand by the lake, children were playing

So а works well because it means something like:

• and meanwhile
• while
• whereas
• and as for...

It is not as simple as и = and, and not as strong as но = but.

So here а feels natural because the sentence moves from one part of the scene to another.

Here it refers to height: the grass was taller or higher.

Russian высокий / выше can be translated differently depending on context:

• высокий дом = a tall building
• высокая гора = a high mountain
• трава выше = the grass is taller

For grass in English, taller is usually the most natural translation.

It is in the nominative plural, because it is the subject of the verb играли.

• singular: ребёнок = child
• plural: дети = children

This is an irregular plural, so learners often need to memorize it separately.

In the sentence:

• играли дети = children were playing

So дети is nominative plural.

Yes. Because this is an imperfective verb, the past tense can often be translated in English as either:

• played
• were playing
• sometimes even used to play, depending on context

Here, because the sentence describes a scene, were playing is usually the most natural translation:

• на песке у озера играли дети = children were playing on the sand by the lake

The verb играли does not itself force one single English tense choice; context helps decide.

It has two main parts joined by а:

1. Рядом с сосной трава была выше

   • Рядом с сосной = location
   • трава = subject
   • была выше = predicate

2. на песке у озера играли дети

   • на песке у озера = location
   • играли = verb
   • дети = subject

So the sentence is basically:

• [location] + [subject] + [verb],
  then
• [location] + [verb] + [subject]

This kind of variation is very normal in Russian.

A few words are worth noticing:

• рядом → stress on the first syllable: РЯ-dom
• сосной → stress on the last syllable: so-sНОЙ
• песке → stress on the second syllable: pe-skЕ
• озера → stress on the second syllable: o-zЁ-ra in careful pronunciation, though spelling is озера without ё
• играли → stress on the second syllable: i-grА-li
• дети → stress on the first syllable: ДЕ-ti

Stress matters a lot in Russian, so it is useful to learn each form with its stress, not just the dictionary spelling.