Я заметил одну мелочь в отчёте.

Заметил is past tense: noticed.

It also shows that the speaker is masculine. In Russian past tense, verbs agree in gender and number:

• я заметил = I noticed (spoken by a man)
• я заметила = I noticed (spoken by a woman)
• мы заметили = we noticed

So if a woman said this sentence, it would be:

• Я заметила одну мелочь в отчёте.

This is about aspect, which is very important in Russian.

• заметить → perfective
• замечать → imperfective

Я заметил... uses the perfective past, which means the action is seen as completed: at some point, I noticed it.

That fits this sentence well, because the speaker is talking about a specific result: they spotted one detail.

By contrast, я замечал would usually suggest something like:

• I used to notice
• I was noticing
• I had noticed on various occasions

So here заметил is the natural choice.

Because мелочь is the direct object of the verb заметил, so it goes into the accusative case.

The basic form is:

• одна мелочь = one small thing / one detail

In the accusative singular feminine, одна changes to одну:

• одна → одну

So:

• Я заметил одну мелочь = I noticed one detail / one small thing

Because мелочь is a feminine noun of the 3rd declension ending in -ь, and in this declension the nominative singular and accusative singular are often the same.

So:

• nominative: мелочь
• accusative: мелочь

That is why only одна → одну changes visibly, while мелочь stays мелочь.

You mostly learn it from the dictionary form, but there is a useful pattern.

Nouns ending in -ь can be either masculine or feminine, so the ending alone does not always tell you the gender. However:

• many feminine nouns ending in -ь belong to the 3rd declension
• мелочь is one of them

Other common feminine nouns of this type include:

• ночь = night
• дверь = door
• мысль = thought

So мелочь is feminine, which is why you get:

• одна мелочь
• одну мелочь

Here мелочь does not mean a literally tiny physical object. It usually means something like:

• a small detail
• a minor point
• a little issue
• a trivial thing

In the sentence Я заметил одну мелочь в отчёте, it most naturally means:

• I noticed one small detail in the report
• I noticed one minor issue in the report

The exact English translation depends on context. If the speaker is being critical, it can sound like a small problem. If they are being neutral, it can simply mean a small detail.

Because в can take different cases depending on the meaning.

Here it means in the report, describing location, so Russian uses the prepositional case:

• в отчёте = in the report

Compare:

• в отчёте = in the report, inside the report
• в отчёт = into the report

So if you are talking about where something is found, в отчёте is correct.

The letter ё is always pronounced yo and is always stressed.

So:

• отчёт sounds roughly like at-CHYOT
• в отчёте sounds roughly like v at-CHYO-tye

A useful thing to know: in many Russian texts, ё is often written as е, even though the pronunciation is still ё. So you may sometimes see:

• в отчете

but it is still pronounced:

• в отчёте

It can do a bit of both, depending on context.

Literally, одну means one. But Russian has no articles like a/an/the, so words like один / одна / одно can sometimes sound a little like a certain or one in English.

In this sentence, одну мелочь most naturally means:

• one detail
• one little thing

It may emphasize that the speaker noticed just one specific point.

If you remove одну, you get:

• Я заметил мелочь в отчёте.

That is possible, but it sounds a bit different and less natural in many contexts. Одну helps make the statement more specific.

Yes. Russian word order is flexible, and changing it usually changes the focus or emphasis, not the basic meaning.

For example:

• Я заметил одну мелочь в отчёте.
  Neutral: I noticed one detail in the report.

• В отчёте я заметил одну мелочь.
  Emphasis on in the report.

• Одну мелочь я заметил в отчёте.
  Emphasis on one detail.

The original sentence is a very natural neutral order.

Sometimes, but here you should be careful.

In Russian, pronouns are often omitted when the subject is clear from context. However, in the past tense, the verb does not show person clearly:

• заметил can mean I noticed or he noticed, depending on context

So:

• Я заметил одну мелочь в отчёте. is clear on its own
• Заметил одну мелочь в отчёте. can work in conversation, but only if the context already makes it obvious who noticed it

So yes, Я can be dropped in the right context, but keeping it is safer and clearer.

You could, but the meaning shifts a little.

• заметил = noticed
• увидел = saw

Заметил focuses on becoming aware of something, especially a detail or issue. That is why it works very well with мелочь.

• Я заметил одну мелочь в отчёте.
  I noticed one small detail in the report.

If you say:

• Я увидел одну мелочь в отчёте.

it sounds less natural, because увидеть is more about literally seeing something, while заметить is more about noticing or spotting it. For reports, documents, mistakes, and details, заметить is usually the better choice.