Потом я вставила адрес в другое письмо и проверила отправителя и получателя.

Because the verb is in the past tense, and in Russian past tense verbs agree with the subject in gender and number.

• я вставила = I inserted (said by a woman)
• я вставил = I inserted (said by a man)

So this sentence suggests that the speaker is female.

Both verbs refer to the same subject: я.

So if the speaker is female, both past-tense verbs must match:

• я вставила
• я проверила

Russian past tense works this way:

• masculine: вставил
• feminine: вставила
• neuter: вставило
• plural: вставили

These are perfective verbs because the sentence describes completed actions:

• вставила = inserted / pasted in
• проверила = checked

The sentence presents the actions as finished steps in a sequence:

1. Потом she inserted the address
2. and checked the sender and recipient

If you used imperfective forms such as вставляла or проверяла, it would sound more like an ongoing process, repeated action, or background action rather than a completed result.

Адрес is in the accusative case because it is the direct object of вставила.

But for many inanimate masculine nouns in Russian, the accusative singular looks exactly like the nominative singular.

So:

• nominative: адрес
• accusative: адрес

That is why there is no visible change here.

Because в can take different cases depending on the meaning.

Here it means into, showing movement or transfer into something, so it takes the accusative:

• в другое письмо = into another letter/email

If it meant location, in or inside, then it would take the prepositional:

• в другом письме = in another letter/email

So:

• в + accusative = into / to
• в + prepositional = in / inside

Because письмо is a neuter noun.

The adjective другой must agree with it in gender, number, and case.

Here we have:

• письмо = neuter, singular
• accusative singular (same form as nominative for an inanimate noun)

So the adjective must also be neuter singular:

• другое письмо

Compare:

• другой человек = another person (masculine)
• другая книга = another book (feminine)
• другое письмо = another letter/email (neuter)

They are in the accusative singular because they are the direct objects of проверила.

However, both nouns refer to people, so they are animate masculine nouns. In Russian, the accusative singular of animate masculine nouns usually looks like the genitive singular.

So:

• nominative: отправитель → accusative: отправителя
• nominative: получатель → accusative: получателя

This is a very important pattern:

• inanimate masculine: accusative = nominative
• animate masculine: accusative = genitive

That is why адрес stays адрес, but отправитель becomes отправителя.

They are in the accusative case.

Even though they look like genitive forms, in this sentence they function as the direct objects of проверила:

• проверила кого? что?
  • отправителя
  • получателя

Because they are animate masculine nouns, their accusative forms happen to match the genitive.

It can mean either, depending on context.

Literally, письмо means letter, but in modern Russian it is also very commonly used for email.

In this sentence, because of words like адрес, отправитель, and получатель, many learners would naturally understand it as something like:

• another email
• another message
• another letter

So yes, письмо often means email in real usage.

Because Russian often omits a repeated subject when it is already clear.

So:

• Потом я вставила адрес в другое письмо и проверила отправителя и получателя.

means:

• Then I inserted the address into another letter/email and checked the sender and recipient.

Russian does not need to repeat я here, because the same person is doing both actions.

You could say и я проверила, but it would usually sound more emphatic or stylistically different.

Yes. Russian word order is more flexible than English word order.

This sentence is perfectly natural as written:

• Потом я вставила адрес в другое письмо и проверила отправителя и получателя.

But other orders are possible, depending on emphasis. For example:

• Потом я проверила отправителя и получателя и вставила адрес в другое письмо.
• Адрес потом я вставила в другое письмо...

The original order sounds like a natural sequence of actions. Russian word order often helps show what is already known and what is being emphasized, rather than strictly marking grammar.

Вставить means to insert, put in, or in many digital contexts paste/insert.

In this sentence, вставила адрес most naturally suggests:

• inserted the address
• pasted the address into another email/message

Other verbs would give different meanings:

• ввести адрес = enter/type in the address
• вклеить = literally paste/glue in, or figuratively paste in, but less neutral here
• добавить адрес = add the address

So вставила is a good choice if the idea is placing existing text into another message.

Потом means then, after that, or later. It often appears at the beginning of a sentence to show sequence.

Here it signals that this action happened after something earlier:

• Потом я вставила... = Then I inserted...

You can move потом in Russian, but sentence-initial position is very common and natural.

Only the past-tense verb endings would change:

• Потом я вставил адрес в другое письмо и проверил отправителя и получателя.

So:

• female speaker: вставила, проверила
• male speaker: вставил, проверил

Everything else stays the same.