Мы нашли большой муравейник у сосны и решили не подходить близко.

Нашли is the past tense of the perfective verb найти, which means to find in the sense of a completed result.

• мы нашли = we found
• мы находили would usually mean we used to find, we were finding, or we found on multiple occasions, depending on context

In this sentence, the speaker is talking about one completed event, so нашли is the natural choice.

Because муравейник is:

• masculine
• singular
• inanimate

In Russian, masculine singular inanimate nouns have the same form in the accusative as in the nominative.

So:

• nominative: большой муравейник
• accusative: большой муравейник

That is why nothing changes here, even though it is the direct object of нашли.

У сосны means by the pine tree, near the pine tree, or next to the pine tree.

The preposition у often means by / near / at when talking about location.

So:

• у дома = by the house
• у реки = by the river
• у сосны = by the pine tree

It gives the location of the anthill.

Because the preposition у requires the genitive case.

The dictionary form is:

• сосна = pine tree

But after у, it changes to the genitive singular:

• у сосны

So the pattern is:

• у + genitive

Examples:

• у брата = at the brother’s place / by the brother
• у дороги = by the road
• у сосны = by the pine tree

Russian does not have articles like English a and the.

So большой муравейник can mean:

• a big anthill
• the big anthill

Which one is meant depends on context. In this sentence, English would usually say a big anthill, but Russian simply leaves that unstated.

This means decided not to come close / not to approach too closely.

Breakdown:

• решили = decided
• не = not
• подходить = to approach / to come up to
• близко = closely / near

So literally it is something like:

• we decided not to approach near

More natural English is:

• we decided not to get too close

After решили (decided), Russian commonly uses an infinitive to say what someone decided to do or not do.

Pattern:

• решить + infinitive = to decide to do something

Examples:

• решили уйти = decided to leave
• решили подождать = decided to wait
• решили не подходить = decided not to approach

This works very much like English decided to...

This is a good aspect question. Both can be possible in Russian, but they give slightly different shades of meaning.

• не подходить uses the imperfective
• не подойти uses the perfective

Here, не подходить близко sounds natural because it focuses on the action in a general sense: not go near, not come close.

If you said решили не подойти близко, it would sound more like decided not to come up close even once / not to make that move.

So:

• не подходить = not approach, not go near
• не подойти = not come up to, not make a single approach

In this sentence, the imperfective is very natural because the idea is simply avoiding getting close.

Because близко is an adverb, and it modifies the verb подходить.

• близкий = close as an adjective
• близко = closely / near as an adverb

Here we need an adverb because the sentence describes how they would approach.

Compare:

• близкий друг = a close friend
• подойти близко = to come close

Because the subject is the same in both parts of the sentence.

• Мы нашли ...
• и решили ...

Russian, like English, often omits the repeated subject when it is obvious.

English does the same:

• We found a big anthill and decided not to get close.

You do not need to say we again before decided.

Because the subject is мы (we), which is plural.

Past tense in Russian agrees with the subject in number and, in the singular, also gender.

So:

• я решил / решила = I decided
• он решил = he decided
• она решила = she decided
• мы решили = we decided

The same logic applies to нашли:

• мы нашли = we found

Yes, Russian word order is fairly flexible.

The basic neutral order here is:

• Мы нашли большой муравейник у сосны и решили не подходить близко.

But other orders are possible depending on emphasis, for example:

• У сосны мы нашли большой муравейник и решили не подходить близко.
• Большой муравейник мы нашли у сосны и решили не подходить близко.

These all mean roughly the same thing, but the focus changes slightly.

The original sentence sounds natural and neutral.

Муравейник means anthill or ant nest.

It comes from муравей (ant).

So:

• муравей = ant
• муравейник = anthill

In this sentence, it refers to the mound or nest of ants they found near the pine tree.

In practice, it tells you where the anthill was found, so it can be understood as locating the anthill: a big anthill by the pine tree.

Russian often allows this kind of phrase to work naturally with the whole idea rather than forcing a sharp distinction.

So the sentence is understood as:

• We found a big anthill that was by the pine tree
• or simply
• We found a big anthill near the pine tree

Both reflect the same basic meaning.