Этот день стал кошмаром.

Because день is a masculine singular noun.

The demonstrative этот must agree with the noun in gender, number, and case:

• этот = masculine singular
• эта = feminine singular
• это = neuter singular
• эти = plural

Since день is masculine, you say этот день.

Because день is the subject of the sentence.

In Этот день стал кошмаром, the thing doing the “becoming” is this day, so it stays in the nominative:

• Этот день = the subject
• стал = became
• кошмаром = a nightmare

Russian usually keeps the subject in the nominative, just as English does.

Стал is the past tense of стать, which means to become.

So the structure is:

• стать + instrumental = to become + something

Examples:

• Он стал врачом. = He became a doctor.
• Всё стало ясно. = Everything became clear.
• Этот день стал кошмаром. = This day became a nightmare.

In this sentence, стал is masculine singular because it agrees with день, which is a masculine singular noun.

In the past tense, Russian verbs agree with the subject in gender and number.

Here the subject is день, which is masculine singular, so you use:

• стал = masculine
• стала = feminine
• стало = neuter
• стали = plural

Compare:

• День стал кошмаром.
• Ночь стала кошмаром.
• Утро стало кошмаром.
• Эти дни стали кошмаром.

Because after стать (to become), Russian normally uses the instrumental case for the thing someone or something becomes.

So:

• кошмар = nominative
• кошмаром = instrumental singular

This is a very common pattern:

• стать учителем = to become a teacher
• стать проблемой = to become a problem
• стать кошмаром = to become a nightmare

Even though English says became a nightmare, Russian marks a nightmare with the instrumental case.

Not here.

It is true that the instrumental case can sometimes express means or accompaniment, but the instrumental has many uses. One of them is after verbs like стать.

So in this sentence, кошмаром does not mean with a nightmare. It means a nightmare, but in the case required by стать.

A useful idea is:

• don’t try to force one English meaning onto each case
• learn common verb + case patterns

Here the pattern is simply:

• стать кем? чем? = to become someone/something

Yes, Этот день был кошмаром is also possible, but the meaning is slightly different.

• Этот день был кошмаром = This day was a nightmare.
• Этот день стал кошмаром = This day became a nightmare.

So:

• был describes a state
• стал emphasizes a change into that state

If you want the sense that the day turned into something terrible, стал кошмаром is especially natural.

Russian has no articles like a, an, or the.

That means Russian relies on context, word order, and words like этот to show definiteness.

Here:

• этот день clearly means this day
• кошмаром can be understood as a nightmare or the nightmare, depending on context, but in English a nightmare is the natural translation

So learners often have to mentally supply the English article when translating.

Yes. Russian word order is more flexible than English because case endings show grammatical roles.

The neutral order here is:

• Этот день стал кошмаром.

But you could also hear variations like:

• Кошмаром стал этот день.
• Стал кошмаром этот день.

These versions change the emphasis, not the basic meaning.

In normal, neutral speech, Этот день стал кошмаром is the most straightforward order.

A careful pronunciation is approximately:

Etat' dyen' stal kash-MA-rom

A few key points:

• Этот is pronounced roughly EH-tət or EH-tat depending on speech style
• день has a soft д and soft н, something like dyen'
• стал is close to stahl
• кошмаром has stress on ма: каш-МА-рам

Stress matters in Russian, so it is useful to remember:

• Э́тот день стал кошма́ром.

It can absolutely mean something generally horrible, not only a literal nightmare during sleep.

So Этот день стал кошмаром very naturally means that the day turned into an awful, unbearable experience.

This is similar to English, where nightmare can also be metaphorical:

• The trip was a nightmare.
• The meeting was a nightmare.

Russian uses кошмар in the same broader way.

It answers the instrumental-case question:

• кем? чем? = as whom? as what? / what did it become?

So:

• Этот день стал чем?
  Кошмаром.

This is a useful way Russians often think about case usage: by linking a verb to the case questions it requires.

For стать, the pattern is:

• стать кем? чем?

So you can build many similar sentences:

• Он стал другом.
• Проблема стала серьёзной.
• Игра стала кошмаром.