На диктанте я ошибся в одном слове, потому что не заметил безударную гласную.

На диктанте means during the dictation or in the dictation exercise/class situation.

Russian often uses на with events, activities, lessons, and organized situations:

• на уроке — in class / during the lesson
• на экзамене — during the exam
• на лекции — at the lecture
• на диктанте — during the dictation

So here на диктанте does not literally mean being physically on top of something. It means the mistake happened while taking the dictation.

In this sentence, я ошибся means I made a mistake or I got something wrong.

The verb is ошибиться. It is very common in Russian for saying that someone made a mistake.

• Я ошибся. — I made a mistake. / I was mistaken.
• Ты ошибся. — You made a mistake.

So yes, it can sometimes translate as was mistaken, but in this sentence the natural English meaning is made a mistake.

The -ся is part of the verb ошибиться. Many Russian verbs have this ending, and it does not always have a simple one-word English equivalent.

Here you should learn ошибиться as a whole verb meaning:

• to make a mistake
• to be mistaken

It is not something you should try to translate word-for-word every time. In this sentence, ошибся is just the normal past tense form of that verb.

В одном слове is in the prepositional case.

The phrase ошибиться в чём-то means:

• to make a mistake in something

Examples:

• ошибиться в слове — to make a mistake in a word
• ошибиться в имени — to make a mistake in a name
• ошибиться в вычислениях — to make a mistake in the calculations

So:

• в
  • prepositional
• одном слове = in one word

Yes. В одном слове means in one word.

Because слове is prepositional singular, the numeral/adjective-like word один also changes form:

• nominative: одно слово
• prepositional: в одном слове

So одном agrees with слове in case, gender, and number.

Russian has no articles like a or the.

Meaning is understood from context.

So в одном слове naturally means in one word. In another context, a noun without an article might be understood as a or the, depending on what makes sense.

That is normal in Russian, and learners gradually get used to it.

Потому что means because.

It introduces the reason:

• Я ошибся в одном слове, потому что не заметил безударную гласную.
• I made a mistake in one word because I didn’t notice the unstressed vowel.

It is one of the most common ways to express cause in Russian.

Не заметил is the perfective past form of заметить.

Here the speaker is talking about one specific failure to notice something at one particular moment in the dictation. That is why perfective fits well.

• не заметил — did not notice
• не замечал — was not noticing / did not used to notice / failed to notice repeatedly, depending on context

In this sentence, the meaning is a single completed situation, so не заметил is the natural choice.

Russian past tense verbs agree with gender in the singular.

• я заметил — I noticed, if the speaker is male
• я заметила — I noticed, if the speaker is female

The same is true for ошибся:

• male: я ошибся
• female: я ошиблась

So this sentence is said by a male speaker. A female speaker would say:

На диктанте я ошиблась в одном слове, потому что не заметила безударную гласную.

It means an unstressed vowel.

Breakdown:

• без- — without
• ударение — stress
• безударный — unstressed
• гласная — vowel

So безударная гласная is literally an unstressed vowel.

This is a very common idea in Russian spelling, because unstressed vowels are often harder to hear clearly.

Because it is the direct object of не заметил.

The speaker did not notice what?
безударную гласную

So the phrase is in the accusative singular feminine:

• nominative: безударная гласная
• accusative: безударную гласную

Both words change because the adjective must agree with the noun.

Yes, very often гласная is shorthand for гласная буква or simply means vowel in context.

Russian uses:

• гласный / гласная — vowel
• согласный / согласная — consonant

In school and spelling contexts, Russians very naturally say things like:

• проверить безударную гласную
• найти гласную
• пропустить согласную

So this is completely normal usage.

Yes, Russian word order is flexible.

This sentence has a very neutral, natural order:

На диктанте я ошибся в одном слове, потому что не заметил безударную гласную.

But you could change the order for emphasis. For example:

• Я ошибся на диктанте в одном слове, потому что не заметил безударную гласную.
• Потому что не заметил безударную гласную, я ошибся в одном слове на диктанте.

These are grammatically possible, but the original sounds very natural and straightforward.

Yes, ошибся в одном слове sounds very natural.

Russian often prefers the verb ошибиться instead of a literal structure like make a mistake.

Both are possible in some contexts, but:

• я ошибся в одном слове — very natural
• я сделал ошибку в одном слове — understandable, but less idiomatic here

A very common alternative would be:

• Я допустил ошибку в одном слове. — I made a mistake in one word.

But in everyday speech, ошибся is especially common.

In Russian, vowels are often reduced when they are unstressed, so the sound may be less clear than the spelling.

That means when writing from dictation, a student may hear a word but still be unsure which vowel letter to write in an unstressed position.

So the sentence implies something like:

• the speaker wrote a word incorrectly
• the reason was that they failed to notice or identify the unstressed vowel correctly

This is a very typical kind of school spelling mistake in Russian.

The main stresses are:

• на дикта́нте
• я ошибся́
• в одно́м слове
• потому́ что
• не заме́тил
• безуда́рную гла́сную

A rough pronunciation guide:

• На дикта́нте — na dik-TAN-tye
• я ошибся́ — ya a-SHIB-sya
• в одно́м сло́ве — v ad-NOM SLO-vye
• потому́ что — pa-ta-MOO shta
• не заме́тил — nye za-MYE-til
• безуда́рную гла́сную — bye-zoo-DAR-noo-yu GLAS-noo-yu

The most important thing for a learner is to remember where the stress falls, because stress in Russian is unpredictable and affects pronunciation a lot.