Почтальон пришёл рано утром и оставил газету в почтовом ящике.

Почтальон is the subject of the sentence, so it is in the nominative case, singular.

This noun is masculine, and many masculine Russian nouns have no visible ending in the nominative singular, especially when they end in a consonant:

• почтальон
• студент
• дом

So here почтальон simply means the postman / mail carrier as the subject of the action.

Пришёл is the past tense, masculine singular form of the verb прийти/приходить meaning to come / to arrive.

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

• он пришёл — he came
• она пришла — she came
• оно пришло — it came
• они пришли — they came

Since почтальон is masculine singular, the verb is пришёл.

Also, the letter ё is important here because it shows the stress: пришёл. In everyday writing, Russians often write е instead of ё, so you may also see пришел, but it is pronounced пришёл.

The sentence describes two completed actions done by the same person:

1. the postman arrived
2. he left the newspaper

Russian often links actions with и (and) without repeating the subject:

• Почтальон пришёл и оставил...

English would usually also do this:

• The postman came and left...

Both verbs are masculine singular past forms because they both refer to почтальон.

This is a very common Russian time expression.

• рано = early (an adverb)
• утром = in the morning

So рано утром means early in the morning.

The interesting part is утром. It is the instrumental case form of утро, but in expressions of time, Russian often uses forms like this adverbially:

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

So you do not need to think of утром here as a literal instrument; it is just a standard time expression.

They are perfective because the sentence presents single, completed events in sequence.

• пришёл = he arrived
• оставил = he left/placed and the action is complete

This is very natural in narration: one finished action happened, then another finished action happened.

If you used imperfective verbs instead, the meaning would change. It could sound more like repeated action, process, or background:

• приходил — used to come / came by / was coming, depending on context
• оставлял — used to leave / was leaving

So in this sentence, perfective is the normal choice because it tells the story as a completed event.

Because газету is the accusative singular form, and газета is the direct object of оставил.

Dictionary form:

• газета — newspaper

Accusative singular:

• газету

For many feminine nouns ending in -а, the accusative singular changes to -у:

• книга → книгу
• машина → машину
• газета → газету

So:

• оставил что? — газету

Because here the phrase gives the location where the newspaper ended up being left: in the mailbox.

With в, Russian often uses:

• accusative for motion into something
• prepositional for location inside something

Compare:

• положил газету в почтовый ящик — put the newspaper into the mailbox
  (focus on movement into it)
• оставил газету в почтовом ящике — left the newspaper in the mailbox
  (focus on where it was left)

So in your sentence, the emphasis is on the resulting location, not on the movement itself.

Because both words are in the prepositional case, singular, after в in a location meaning.

Basic form:

• почтовый ящик — mailbox

Prepositional form:

• в почтовом ящике — in the mailbox

The adjective must agree with the noun:

• почтовый → почтовом
• ящик → ящике

This is standard adjective-noun agreement in Russian:

• same case
• same number
• same gender

No, Russian word order is fairly flexible. The version in the sentence is a natural, neutral narrative order:

Почтальон пришёл рано утром и оставил газету в почтовом ящике.

But other orders are possible if you want different emphasis:

• Рано утром почтальон пришёл и оставил газету в почтовом ящике.
  Emphasizes the time.
• Газету почтальон оставил в почтовом ящике.
  Emphasizes the newspaper.
• В почтовом ящике почтальон оставил газету.
  Emphasizes the location.

So the words can move, but the choice affects style and focus.

Because Russian does not have articles like English a/an and the.

So:

• почтальон can mean a postman or the postman
• газету can mean a newspaper or the newspaper
• в почтовом ящике can mean in a mailbox or in the mailbox

Russian usually shows definiteness through:

• context
• word order
• what is already known in the conversation

So learners often want to insert an article somewhere, but Russian simply does not use them.

A learner often wants to know what forms to look up in a dictionary. Here are the main ones:

• почтальон → dictionary form is already почтальон
• пришёл → look up прийти or also learn the imperfective partner приходить
• рано → dictionary form is рано
• утром → dictionary form is утро
• оставил → look up оставить; imperfective partner: оставлять
• газету → dictionary form is газета
• почтовом → dictionary form is почтовый
• ящике → dictionary form is ящик

This is very useful because Russian sentences often contain forms that look quite different from the dictionary entry.

Stress is very important in Russian because it is not always predictable. The main stresses here are:

• почтальо́н
• пришёл
• ра́но
• у́тром
• оста́вил
• газе́ту
• почто́вом
• я́щике

A couple of notes:

• ё is always stressed, so пришёл is easy once you see ё
• stress can move when a word changes form, so it is worth learning words together with stress marks when possible