Мне нужно доделать отчёт сегодня, иначе завтра будет поздно.

Because нужно does not work like a normal verb meaning need.

In Russian, this pattern is very common:

• мне нужно = I need
• literally: to me it is necessary

So мне is in the dative case, not the nominative.

Compare:

• Я работаю = I am working
• Мне нужно работать = I need to work

This is one of the most important Russian patterns to get used to: many feelings, states, and necessities are expressed with a dative pronoun plus a predicate word.

Нужно means necessary / needed / have to in this sentence.

So:

• Мне нужно доделать отчёт = I need to finish the report

It is often used with an infinitive:

• Мне нужно уйти = I need to leave
• Нам нужно поговорить = We need to talk

A close synonym is надо:

• Мне надо доделать отчёт = almost the same meaning

Нужно can sound a little more neutral or formal in some contexts, but in everyday speech both are very common.

Доделать means to finish doing something that was already partly done.

That is the key idea here.

• делать = to do, to be doing
• сделать = to do / complete
• доделать = to finish off, complete the remaining part

So in this sentence, доделать отчёт suggests:

• the report has probably already been started
• now it needs to be finished

This makes доделать very natural.

Why not делать?
Because делать is imperfective, and after нужно it would usually mean the process in general, not the completed result:

• Мне нужно делать отчёт = I need to work on the report / be doing the report

Why not сделать?
You could say Мне нужно сделать отчёт сегодня, and it would also be correct. But доделать specifically emphasizes finishing what remains.

Отчёт is the direct object of доделать, so it is in the accusative case.

However, for an inanimate masculine singular noun, the accusative often looks exactly the same as the nominative.

So:

• nominative: отчёт
• accusative: отчёт

That is why you do not see a visible ending change here.

Compare with a feminine noun:

• книга → книгу

Or with an animate masculine noun:

• студент → студента

So the case has changed grammatically, but the form happens to stay the same.

Because сегодня and завтра are adverbs of time.

Russian often uses these time words directly, with no preposition:

• сегодня = today
• завтра = tomorrow
• вчера = yesterday

So:

• Мне нужно доделать отчёт сегодня = I need to finish the report today
• Завтра будет поздно = tomorrow will be too late

This is similar to English, where we also usually say today and tomorrow without a preposition.

Иначе means otherwise or or else.

It introduces the consequence if the first part is not done:

• Мне нужно доделать отчёт сегодня, иначе завтра будет поздно.
• I need to finish the report today, otherwise tomorrow will be too late.

The comma is there because Russian is connecting two clauses:

1. Мне нужно доделать отчёт сегодня
2. иначе завтра будет поздно

Russian punctuation is often stricter than English about separating clauses like this, so the comma is expected.

Because поздно here is not an adjective describing a noun. It is a predicative adverb/state word meaning it is late / too late.

So:

• поздний = late, as an adjective
  
  • поздний поезд = a late train
• поздно = late, as a state
  
  • уже поздно = it’s already late

In this sentence:

• завтра будет поздно = tomorrow it will be too late

You are describing a situation, not a noun.

As for позднее, that usually means later, not late:

• Я приду позднее = I will come later

So будет поздно is exactly the right form here.

There is no normal subject here. This is an impersonal construction.

Russian often says things like:

• было трудно = it was hard
• будет интересно = it will be interesting
• уже поздно = it’s already late

English often uses a dummy subject it, but Russian usually does not need one.

So:

• завтра будет поздно
  literally feels like: tomorrow will be late / too late

More naturally in English: tomorrow will be too late.

Yes. Russian word order is flexible, and different orders change the emphasis more than the basic meaning.

The original sentence is natural:

• Мне нужно доделать отчёт сегодня, иначе завтра будет поздно.

But you could also say:

• Сегодня мне нужно доделать отчёт, иначе завтра будет поздно.
  Emphasis on today.

• Отчёт мне нужно доделать сегодня, иначе завтра будет поздно.
  Emphasis on the report.

• Мне нужно сегодня доделать отчёт, иначе завтра будет поздно.
  Slightly stronger focus on the timing.

So the original order is neutral and very natural, but it is not the only possible one.

Yes, it is natural.

In Russian, доделать is often used for something that is incomplete and needs the final work done:

• доделать проект = finish the project
• доделать домашнее задание = finish the homework
• доделать отчёт = finish the report

It can sound slightly more practical or task-oriented than закончить.

You could also say:

• Мне нужно закончить отчёт сегодня

That also means I need to finish the report today.

The difference is roughly:

• закончить = finish, complete
• доделать = finish the remaining part of something already underway

Yes, the correct spelling is отчёт, with ё.

This matters because:

• ё is pronounced yo
• so отчёт sounds like atchyot (roughly)

In many Russian texts, especially informal ones, ё is often written as е:

• отчет

But it is still pronounced отчёт.

For learners, it is helpful to remember the real form with ё, because it makes pronunciation and stress much clearer.

Sometimes in meaning, yes, but not always in tone.

• Мне нужно доделать отчёт сегодня = I need to finish the report today
• depending on context, it may feel close to I have to or I must

But must in English can sound stronger or more formal.

Russian has several ways to express necessity:

• мне нужно = I need to / it is necessary for me
• мне надо = I need to / have to
• я должен = I must / I am obliged to

So мне нужно is usually best understood as I need to or I have to, rather than the strongest possible I must.