На столе лежал конверт, и я не сразу заметила, что на нём уже написан адрес.

Because на столе means on the table in the sense of location.

• на + accusative often means motion onto something: положить на стол = to put onto the table
• на + prepositional often means location on something: лежать на столе = to be lying on the table

Here the envelope is already there, so Russian uses the location form: на столе.

Russian often prefers specific verbs of position where English might simply say there was or was lying.

• лежать = to lie, to be lying
• стоять = to stand, to be standing
• сидеть = to sit, to be sitting

So На столе лежал конверт literally means An envelope was lying on the table. This sounds natural in Russian and often gives a more vivid picture than just using быть.

Because конверт is the subject of the sentence.

In На столе лежал конверт:

• на столе = adverbial phrase of location
• лежал = verb
• конверт = the thing doing the lying, so grammatically it is the subject

Even though English often starts with An envelope..., Russian can begin with the location first and put the subject later. The subject still stays in the nominative: конверт.

Because the verb in the past tense agrees with the subject in gender and number.

• конверт is masculine singular
• so the past-tense verb is masculine singular: лежал

Compare:

• книга лежала = the book was lying
• письмо лежало = the letter was lying
• конверты лежали = the envelopes were lying

Because past-tense verbs in Russian agree with the subject in gender and number.

Here the subject is я, but in the past tense я does not show gender by itself. The verb tells you:

• я заметил = I noticed, spoken by a man
• я заметила = I noticed, spoken by a woman

So this sentence tells us the speaker is female.

Не сразу means not immediately, not right away, or not at once.

So:

• я не сразу заметила = I didn’t notice immediately
• more naturally in English: I didn’t notice right away

It does not mean not suddenly. It means there was a short delay before the speaker noticed it.

Because Russian is saying on it / on the envelope.

The pronoun refers back to конверт:

• на нём = on it
• more literally: on him/it, but for an inanimate masculine noun like конверт, English just says on it

The case is prepositional because it follows на in a location meaning.

So:

• на конверте = on the envelope
• на нём = on it

Russian personal pronouns often add н- after many prepositions.

Compare:

• он = he / it
• у него = at his place / he has
• к нему = toward him
• на нём = on him / on it

That extra н- is normal after prepositions with third-person pronouns.

Also, you may sometimes see нем written instead of нём because Russian often omits the dots over ё in ordinary writing. But it is pronounced нём here.

Because the sentence is describing a state/result, not focusing on who performed the action.

• написали адрес = someone wrote the address
• написан адрес = the address is written / has been written

Here Russian uses the short-form passive participle написан to say that the address was already there, already written.

So the idea is not someone already wrote the address but rather the address was already written on it.

In Russian, short-form passive participles are often used without был when the state is clear from context.

So:

• на нём уже написан адрес = there is already an address written on it / an address is already written on it
• in this past context, English naturally translates it as there was already an address written on it or the address had already been written on it

Russian does not always need an explicit past form of to be here. The past-time meaning is understood from the surrounding sentence: лежал, заметила.

You could sometimes see был написан, but here the version without был sounds natural and concise.

Because написан agrees with адрес, and адрес is masculine singular.

In Russian, adjectives and participles can come before or after the noun, but they still agree with it:

• написанный адрес = a written address
• адрес написан = the address is written
• на нём написан адрес = an address is written on it

Since адрес is masculine singular, the short-form passive participle is написан.

Russian word order is flexible, and this order sounds natural because it presents the information step by step:

• что = that
• на нём = on it
• уже = already
• написан адрес = an address is written

This order emphasizes the location first: on it. That makes sense because the speaker notices something about the envelope’s surface.

Other word orders are possible, for example что адрес на нём уже написан, but the given version sounds smooth and natural in context.

Because написан here is not a normal past-tense finite verb like лежал or заметила. It is a short passive participle, which mainly describes a resulting state.

So the timeline is:

• the envelope was lying there
• the speaker did not notice right away
• when she finally noticed, the address was already in the state of being written on it

English often uses was written or had already been written, but Russian can express that result-state with написан.

Уже means already. It shows that by the time the speaker noticed the envelope, the address was there beforehand.

Without уже, the sentence would simply say that an address was written on it. With уже, it suggests:

• this was true before the speaker noticed it
• perhaps the speaker expected the envelope to be blank, or only realized later that it had already been addressed

So уже adds a sense of prior completion.

In practice, no. It naturally refers to конверт.

Why?

• The nearest masculine singular noun is конверт
• Semantically, an address is written on an envelope, not on a table

So even though Russian pronouns can sometimes be ambiguous, here the meaning is clear from both grammar and common sense.