У дерева был большой муравейник, и дети решили туда не подходить.

Yes, this is a very common Russian pattern.

У + genitive can mean:

• by / near
• or, in some contexts, someone has

In this sentence, У дерева был большой муравейник literally means something like:

• By the tree there was a big anthill
• or There was a big anthill near the tree

So it is not really saying the tree possessed an anthill in an English sense. It is more about location.

Because the preposition у normally requires the genitive.

Base form:

• дерево = tree

After у:

• у дерева = by the tree / near the tree

This is just a case rule you need to learn with the preposition у.

Because был agrees with муравейник, which is masculine singular.

• муравейник = masculine singular
• so in the past tense: был

Compare:

• был for masculine
• была for feminine
• было for neuter
• были for plural

So:

• У дерева был большой муравейник = There was a big anthill near the tree

If the noun were neuter, then you would use было instead.

Because it is the thing that was there.

In Russian existential-type sentences like this, the noun that exists or is present is often in the nominative:

• У дерева был большой муравейник

Here:

• большой муравейник = the subject-like element
• у дерева = the location

So the structure is basically:

• Near the tree was a big anthill

That is why муравейник and its adjective большой are in the nominative singular masculine form.

Because решить means to decide, and in Russian it is very commonly followed by an infinitive, just like in English.

• дети решили = the children decided
• не подходить = not to approach / not to go near

So:

• дети решили туда не подходить = the children decided not to go near there

This is a very useful pattern:

• Я решил поехать. = I decided to go.
• Мы решили подождать. = We decided to wait.

Because туда means to there or there (with motion toward a place), while там means there (location, no motion).

Compare:

• там = there
• туда = to there / over there / to that place

Since подходить involves movement toward something, туда is the natural choice:

• решили туда не подходить = decided not to go over there / not to approach that place

If you used там, it would sound wrong here because там does not express movement toward a destination.

This is an aspect question.

• подходить = imperfective
• подойти = perfective

After решили не..., Russian often uses the imperfective infinitive when talking about a decision not to do something in a general sense.

So:

• решили туда не подходить = they decided not to go near it / not to approach it

This sounds natural as a general decision or rule for themselves.

Could решили туда не подойти appear? It is much less natural here. The perfective would focus more on a single completed approach, while the imperfective fits the idea of avoiding going near.

Because не usually goes directly before the verb or infinitive that it negates.

Here it negates подходить:

• не подходить = not to approach / not to go near

So:

• решили туда не подходить = decided not to approach there / decided not to go near that place

This is the standard position.

Because туда is a very natural way to refer back to a place that has just been mentioned.

The first part sets the scene:

• У дерева был большой муравейник

Then the second part refers back to that location:

• и дети решили туда не подходить

So туда means something like:

• there
• to that place
• practically, near the anthill / tree area

Russian often avoids repetition this way, just like English might say and the children decided not to go over there.

И simply means and.

It joins the two parts:

1. У дерева был большой муравейник
2. дети решили туда не подходить

So the whole sentence is:

• There was a big anthill near the tree, and the children decided not to go near it/there.

Nothing unusual is happening here; it is a normal coordinating conjunction.

Yes. Russian word order is flexible, and different orders create different emphasis.

Given sentence:

• У дерева был большой муравейник, и дети решили туда не подходить.

This is natural and neutral.

You could also say:

• Большой муравейник был у дерева...

That would put more emphasis on the anthill itself.

Russian often places location first in this kind of sentence:

• У дерева был... which feels very natural when introducing something in a place.

So the original word order is a good standard model.

It can be, but in natural English it often means more like:

• go near
• come close to
• approach

In this sentence, не подходить is probably best understood as:

• not go near it
• not come close to it

That is because the children are avoiding the anthill area, not just performing a formal act of approaching.

So while approach is correct, go near may sound more natural in everyday English.

Because Russian has no articles.

So:

• дерево can mean a tree or the tree
• муравейник can mean an anthill or the anthill

You understand which one is meant from context.

In this sentence:

• У дерева был большой муравейник is naturally understood as There was a big anthill near the tree or near a tree, depending on context.

English forces you to choose an article, but Russian does not.