Мы доделали проект вечером и наконец спокойно поужинали.

Доделали means finished doing / completed something that was already in progress.

The prefix до- often gives the idea of bringing something to the end. So:

• делать = to do, to be doing
• доделать = to finish doing, to complete what was not finished yet
• сделать = to do, to make, to complete

In this sentence, доделали проект suggests that the project was already being worked on, and then they finished it that evening.

So доделали is a little more specific than сделали.

Because the subject is мы = we.

In Russian past tense, verbs agree with the subject in gender/number:

• -л = masculine singular
• -ла = feminine singular
• -ло = neuter singular
• -ли = plural

So:

• мы доделали
• мы поужинали

Both end in -ли because the subject is plural.

Because проект is the direct object of доделали, so it is in the accusative case.

However, for a masculine inanimate noun in the singular, the accusative usually looks the same as the nominative.

So:

• nominative: проект
• accusative: проект

That is why the form does not change.

Вечером is a very common Russian way to say in the evening.

It is the instrumental case form of вечер, but in this kind of expression it functions almost like an adverb of time.

Compare:

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

So вечером is the natural choice here.

В вечер would not normally be used to mean ordinary in the evening.

Because the sentence describes completed actions:

1. they finished the project
2. they had dinner

Russian aspect is very important here.

• доделали is perfective: the project got finished
• поужинали is perfective: the dinner happened as a completed event

This makes the sentence sound like a sequence of finished facts, which matches the meaning well.

In поужинали, the prefix по- makes the verb perfective.

• ужинать = to have dinner, to be having dinner
• поужинать = to have dinner (as a completed event)

So поужинали means they had dinner / they ate dinner.

This is a very common pattern with verbs for meals:

• завтракать → позавтракать = have breakfast
• обедать → пообедать = have lunch
• ужинать → поужинать = have dinner

Here, по- does not mean a little. It mainly marks the action as a complete event.

Спокойно literally means calmly, but in this sentence it is more natural to understand it as:

• peacefully
• without stress
• without rushing
• at ease

So наконец спокойно поужинали suggests that after finishing the project, they were finally able to sit down and have dinner in peace.

It adds an emotional nuance, not just information about manner.

Наконец means finally / at last.

It shows that the speakers had probably been busy for a while, and only after finishing the project could they eat dinner.

So the feeling is:

• first they were occupied
• then the project was finished
• only after that were they able to relax and eat

It gives the sentence a sense of relief.

Russian often does not need an extra word like then if the sequence is already clear.

The structure:

Мы доделали проект вечером и наконец спокойно поужинали

naturally reads as:

• we finished the project in the evening
• and then/finally we had dinner peacefully

The conjunction и plus the use of perfective past verbs is enough to show a sequence of completed actions.

If needed, Russian could add words like потом or затем, but it is not necessary here.

Yes, often it can.

Russian past-tense verbs show number, so доделали and поужинали already tell us the subject is plural. In context, the sentence could simply be:

Доделали проект вечером и наконец спокойно поужинали.

That can sound natural in conversation if the subject is already understood.

However, keeping мы is also perfectly normal. It can make the sentence clearer or slightly more explicit.

Grammatically, it is placed next to the first action, so the most direct reading is that it modifies доделали проект:

• We finished the project in the evening

Then the second action happened after that.

In real understanding, dinner would probably also have been in the evening, but the sentence explicitly attaches вечером to the first verb. The second action is simply presented as the next event.

Russian word order is flexible, and both are possible.

The version here sounds neutral and natural:

Мы доделали проект вечером...

If you say:

Мы вечером доделали проект...

that puts a little more emphasis on when it happened.

So the difference is mostly about focus and information flow, not basic grammar.

Yes, it could be omitted:

Мы доделали проект вечером и наконец поужинали.

That would simply mean:

• we finished the project in the evening and finally had dinner

Adding спокойно gives extra feeling: they were able to eat without pressure, without interruption, in peace.

So it is not required grammatically, but it adds an important nuance to the situation.