Сначала преступление кажется простым, а потом каждая мелочь превращается в загадку.

Because the verb казаться normally takes the thing you seem to be in the instrumental.

So in:

преступление кажется простым

the subject is преступление and the quality assigned to it is простым.

Compare:

• Он кажется уставшим. = He seems tired.
• Эта идея кажется странной. = This idea seems strange.

This is a very common pattern in Russian:

• X кажется Y-им / Y-ой / Y-ым

So простым is not random; it is the expected form after кажется.

The dictionary form is казаться, not казать in this meaning.

• казаться = to seem, to appear
• кажется = it seems / seems

This verb is reflexive-looking because of -ся, but in modern Russian you usually just learn it as the normal verb for to seem.

So:

• преступление кажется простым = the crime seems simple

If you used кажет, that would not mean seems here.

Because преступление is a neuter noun in Russian. You can tell from the ending -ие, which is very often neuter.

That matters because adjectives and past-tense verbs would agree with it in neuter when needed.

For example:

• тяжёлое преступление = a serious crime
• преступление было странным = the crime was strange

In your sentence, преступление is the subject of кажется.

Because каждая means each / every, and in Russian that is followed by a singular noun.

So:

• каждая мелочь = every little detail / every small thing

This is just like:

• каждый человек = every person
• каждая книга = every book
• каждое слово = every word

Even though English sometimes feels conceptually plural, Russian keeps it singular after каждый / каждая / каждое.

Literally, мелочь can mean something small, minor, trivial, or a little detail. In other contexts it can even mean small change (coins).

Here it means something like:

• small detail
• little thing
• minor point

So каждая мелочь is a very natural way to say every little detail.

It gives the idea that even things that seemed unimportant start to become mysterious.

Because превращаться в + accusative is the normal pattern for turn into / transform into.

So:

• превращаться в загадку = to turn into a mystery
• превращаться в проблему = to turn into a problem
• превращаться в кошмар = to turn into a nightmare

The noun after в is in the accusative case because the expression means movement/change into a new state.

Here:

• загадка is the dictionary form
• в загадку is the accusative singular

Превращается comes from превращаться, which is imperfective.

That fits the sentence well because the speaker is describing how things unfold in a general or ongoing way:

• first the crime seems simple,
• then every detail starts turning into a mystery.

The imperfective gives a process-like feel, not one single completed transformation.

The perfective partner is usually превратиться:

• превратилась в загадку = turned into a mystery

But here the sentence is describing a pattern or narrative progression, so превращается is the natural choice.

This pair organizes the sentence in time:

• сначала = at first
• а потом = and then / but then / later

It creates a contrast between two stages:

1. Сначала преступление кажется простым
2. а потом каждая мелочь превращается в загадку

The а here is important. It often marks contrast or a shift, not just simple addition. So the feeling is:

• at first it looks easy,
• but then everything gets complicated.

Yes, Russian word order is flexible, but this version is very natural.

Current order:

Сначала преступление кажется простым, а потом каждая мелочь превращается в загадку.

This puts the time markers first:

• Сначала
• а потом

That helps structure the sentence clearly and dramatically.

You could change the order for emphasis, for example:

• Преступление сначала кажется простым...

That is also correct, but it shifts the focus slightly toward преступление. The original sounds smooth and narrative, especially in storytelling.

Russian has no articles like a or the.

So:

• преступление can mean a crime or the crime
• загадку can mean a mystery or sometimes the mystery, depending on context

Russian relies on context, word order, and the situation to show whether something is definite or indefinite.

In your sentence, English might naturally say the crime because the context probably already identifies it, but Russian does not need a special word for that.

Not exactly.

• загадка = puzzle, riddle, mystery that is hard to explain or solve
• тайна = secret, something hidden

In this sentence, загадка is better because the idea is that each detail becomes puzzling, mysterious, hard to understand.

If you used тайна, it would sound more like each detail becomes a secret, which is a different nuance.

A natural rhythm would highlight the contrast words and the key nouns:

Снача́ла преступле́ние ка́жется просты́м, а пото́м ка́ждая ме́лочь превраща́ется в зага́дку.

Useful stress points:

• снача́ла
• преступле́ние
• ка́жется
• просты́м
• пото́м
• ка́ждая
• ме́лочь
• превраща́ется
• зага́дку

Notice also that что is not present here; the sentence is compact and direct, which is very common in Russian narrative style.