Это не сложная загадка, а попросту невнимательность.

Because this is the very common correction pattern не X, а Y = not X, but rather Y.

So:

• не сложная загадка, а попросту невнимательность
• literally: not a difficult riddle, but simply carelessness/inattention

А is preferred when the second part replaces or corrects the first idea.
Но usually means a more general but/however, not this kind of direct correction.

A useful pattern to remember:

• Это не чай, а кофе. = It’s not tea, it’s coffee.
• Он не врач, а учитель. = He’s not a doctor, he’s a teacher.

In Russian, the verb to be is normally omitted in the present tense.

So English:

• This is not a difficult riddle, but simply carelessness.

becomes Russian:

• Это не сложная загадка, а попросту невнимательность.

There is no present-tense есть here. Using есть in this sentence would sound unnatural.

Russian often works like this in the present tense:

• Он студент. = He is a student.
• Это проблема. = This is a problem.

They are both predicate nouns in a sentence identifying what это is. With это in this kind of present-tense equational sentence, the nominative is very normal.

So:

• загадка = nominative singular
• невнимательность = nominative singular

A learner may expect the instrumental case because Russian often uses instrumental with predicate nouns, especially with forms of быть in other tenses:

• Это была загадка.
• Он стал врачом.

But in present-tense patterns with это, nominative is standard and natural:

• Это ошибка.
• Это не шутка.
• Это невнимательность.

Because сложная is simply an adjective agreeing with загадка, and загадка is nominative feminine singular here.

Agreement:

• загадка = feminine, singular, nominative
• сложная = feminine, singular, nominative

So the adjective must match the noun.

Compare:

• сложная загадка = nominative
• сложной загадки = genitive
• сложной загадкой = instrumental

In this sentence, nominative is required because the noun phrase is part of the main identification structure: This is not a difficult riddle...

Попросту means something like simply, plainly, just, or nothing more than. It often adds a slightly stronger or more dismissive tone than plain просто.

So:

• а попросту невнимательность
  = but simply / plainly just carelessness

It suggests: there’s no mystery here; the explanation is much simpler.

In many contexts, просто could replace it:

• а просто невнимательность

But попросту can sound a bit more emphatic, literary, or pointed.

It can function like either, depending on the English translation.

In Russian, это often points to a situation, result, or thing just mentioned. In English, that may come out as:

• this
• it
• that

So in context, the sentence could be translated as:

• This is not a difficult riddle, but simply carelessness.
• It’s not a difficult riddle, but simply carelessness.

Russian это is very common in sentences where you identify or comment on something.

In Russian, не usually goes directly before the word or phrase being negated. Here it negates the characterization сложная загадка.

So the structure is:

• Это не сложная загадка, а ...
• This is not a difficult riddle, but ...

The important contrast is between two interpretations:

1. сложная загадка
2. невнимательность

This is exactly why the pattern не X, а Y works so well here.

Note that in Russian, placing не before the adjective in a phrase like this is normal and does not mean only the adjective is negated in a narrow way. The whole phrase is being contrasted with the second noun.

Because а is a coordinating conjunction, and Russian normally places a comma before it when it connects two contrasting parts of a sentence.

Here the sentence has a clear contrast:

• не сложная загадка
• а попросту невнимательность

So the comma is required:

• Это не сложная загадка, а попросту невнимательность.

This is standard punctuation for the не X, а Y pattern.

Yes, Russian word order is flexible, but changing it changes the emphasis.

The original:

• Это не сложная загадка, а попросту невнимательность.

puts the contrast in a very natural, balanced way: first rejecting one interpretation, then giving the correct one.

Possible alternative:

• Это попросту невнимательность, а не сложная загадка.

This version foregrounds попросту невнимательность first and then adds the correction afterward. It is still natural, but the emphasis is a little different.

So the original order is especially good when you want the classic not X, but Y structure.

It depends on context. Grammatically, невнимательность is a noun meaning lack of attention.

Possible English renderings include:

• inattention
• carelessness
• lack of attention
• sometimes oversight

Which one sounds best depends on what happened. If someone missed something obvious, English may naturally say carelessness or inattention.

For a learner, the important point is that it names a simple human mistake or failure to pay attention, which fits the contrast with сложная загадка.

It is mostly neutral, but попросту gives it a slightly expressive or rhetorical flavor.

Without попросту, the sentence would sound plainer:

• Это не сложная загадка, а невнимательность.

With попросту, it sounds more like:

• It’s not some difficult puzzle; it’s simply carelessness.

So the sentence is not highly formal, but it does have a mild stylistic emphasis.