Я открыл недописанный отчёт и решил дописать его вечером.

Because открыл is the perfective past form of открыть, and it presents the action as a completed single event: I opened it.

In this sentence, the speaker is talking about a sequence of finished actions:

1. Я открыл недописанный отчёт — I opened the unfinished report.
2. и решил... — and decided...
3. дописать его вечером — to finish writing it in the evening.

If you used открывал, that would usually suggest an ongoing, repeated, or background action, such as I was opening or I used to open.

So here, открыл fits the idea of one completed action in a storyline.

For the same aspect reason: решил is perfective, meaning made the decision.

So я решил дописать его вечером means I decided to finish it in the evening — the decision itself is treated as a completed moment.

If you said решал, it would mean something more like:

• I was deciding
• I spent time deciding
• I kept trying to decide

That is not the meaning here. The sentence describes a clear, completed choice.

Недописанный is a past passive participle used like an adjective.

It comes from the verb дописать / дописывать, meaning to finish writing or to write the rest.
So недописанный отчёт means a report that has not been finished / not fully written.

Grammatically, it agrees with отчёт:

• недописанный — masculine
• singular
• accusative, matching an inanimate masculine noun

Since masculine inanimate accusative looks like the nominative, the form stays недописанный.

Not exactly.

Usually, недописанный written as one word means unfinished, not fully completed, and is treated as a standard lexical item.

Не дописанный as two words is less common and usually appears when the speaker is emphasizing the negation more directly, often with dependent words. For example:

• не дописанный мной отчёт — the report not finished by me

In your sentence, недописанный отчёт is the natural standard choice.

A rough intuition:

• недописанный = unfinished, incomplete
• не дописанный = not finished, with stronger emphasis on the negation or in a specific contrast

Yes, they are closely related, and that is completely natural.

• недописанный отчёт = an unfinished report
• дописать его = to finish writing it

So the sentence has a nice logical flow:

• I opened a report that was not finished
• and decided to finish it

Russian often uses related verb forms like this very naturally.

Because решить commonly takes an infinitive to express deciding to do something.

Pattern:

• решить + infinitive

Examples:

• Я решил уйти. — I decided to leave.
• Она решила позвонить. — She decided to call.
• Мы решили дописать отчёт. — We decided to finish writing the report.

So решил дописать simply means decided to finish writing.

Because the speaker means to complete the writing.

The verb pair is:

• дописывать — imperfective
• дописать — perfective

Here, after решил, the perfective infinitive is very natural because the intention is goal-oriented: finish it.

Compare:

• решил дописать отчёт — decided to finish the report
• решил дописывать отчёт по вечерам — decided to keep working on / continue writing the report in the evenings

So дописать focuses on the completed result.

Его refers back to отчёт.

So:

• отчёт — report
• дописать его — to finish writing it

Here его is the accusative form of the pronoun he/it, and for masculine inanimate nouns in English we translate it as it.

Russian often uses pronouns like this to avoid repeating the noun:

• Я открыл отчёт и решил дописать его вечером.

Without его, the sentence would sound incomplete.

Because Russian often uses the instrumental case to express time periods like in the morning, in the afternoon, in the evening, at night.

So:

• утром — in the morning
• днём — in the daytime / during the day
• вечером — in the evening
• ночью — at night

Therefore:

• дописать его вечером = finish writing it in the evening

Using в вечер would generally not be the normal way to say this.

It is the instrumental case.

The noun is:

• вечер — evening

Instrumental singular:

• вечером

In this sentence, the instrumental is being used adverbially to indicate time when.

So even though it looks like a noun form, it functions a lot like an adverb here: in the evening.

It is in the accusative case, because it is the direct object of открыл.

However, for a masculine inanimate singular noun, the accusative form is the same as the nominative:

• nominative: отчёт
• accusative: отчёт

So the form does not change, even though the case function does.

Because it agrees with отчёт, which is:

• masculine
• singular
• accusative inanimate

The participle/adjective therefore appears in the matching form:

• недописанный отчёт

If the noun changed, the participle would change too:

• недописанная статья — an unfinished article
• недописанное письмо — an unfinished letter
• недописанные документы — unfinished documents

So the ending reflects agreement with the noun.

Not necessarily.

It only means that the report was unfinished. The sentence does not specify who had been writing it.

So it could mean:

• the speaker had started it and had not finished it
• someone else had started it
• the report simply existed in an unfinished state

If Russian wants to specify the person, it can add that information:

• недописанный мной отчёт — the report unfinished by me
• недописанный им отчёт — the report unfinished by him

Without that, the sentence stays neutral.

Yes, Russian word order is flexible.

The version in your sentence is neutral and natural:

• Я открыл недописанный отчёт и решил дописать его вечером.

You could also say:

• Я открыл недописанный отчёт и вечером решил дописать его.
• Я открыл недописанный отчёт и решил вечером дописать его.

But these can shift emphasis slightly.

The original order puts вечером at the end, which often sounds smooth and natural, with the time expression added after the plan itself.

So the default reading is:

• I opened the unfinished report and decided to finish it in the evening.

In normal Russian writing, ё is often replaced by е, so you will very often see:

• отчет

instead of:

• отчёт

Both represent the same word.
The version with ё is more explicit about pronunciation and stress.

For learners, отчёт is helpful because it clearly shows:

• the stressed syllable
• that the vowel is ё

But in real texts, е is extremely common.

Yes, very much so.

The sentence moves through a chain of events:

• Я открыл... — I opened...
• и решил... — and decided...
• дописать его вечером — to finish it later in the evening

Russian often uses perfective past verbs like this to narrate events step by step.

So this sentence is a good illustration of how Russian tells a simple completed storyline.