Пожалуйста, приложите файл к письму.

Приложить is the infinitive, meaning to attach / to enclose.

Приложите is the imperative form: attach! or, with пожалуйста, please attach.

So in this sentence:

• приложить = to attach
• приложите = attach / please attach

Russian usually uses the imperative when giving instructions or making polite requests.

Here пожалуйста means please.

It makes the request more polite:

• Приложите файл к письму. = Attach the file to the email/letter.
• Пожалуйста, приложите файл к письму. = Please attach the file to the email/letter.

A useful thing to know is that пожалуйста can also mean you're welcome in other contexts, but in this sentence it clearly means please.

The ending -ите here shows an imperative addressed to:

• more than one person, or
• one person politely (formal you)

So приложите can mean:

• attach (speaking to several people), or
• please attach (speaking politely to one person)

This is connected to вы-style politeness in Russian.

Compare:

• приложи = attach! (informal, singular)
• приложите = attach! / please attach! (formal or plural)

Because файл is the direct object of the verb приложите.

You are attaching what?
→ файл

For many masculine inanimate nouns, the accusative singular is the same as the nominative singular, so:

• nominative: файл
• accusative: файл

That is why the word does not visibly change here.

Because the preposition к requires the dative case.

The noun письмо is neuter, and its dative singular form is письму.

So:

• письмо = nominative
• к письму = to the letter / to the email

This is a very common pattern in Russian:

• к столу = to the table
• к дому = to the house
• к письму = to the letter/email

Here к means something like to in the sense of attached to or added to.

So приложить файл к письму is literally close to:

• attach a file to a letter/email
• add a file to a message

This is the normal construction with приложить:

• приложить что? = attach what?
• к чему? = to what?

So:

• приложить файл к письму
• приложить документ к заявлению
• приложить фотографию к анкете

It can mean either, depending on context.

In modern everyday Russian, письмо often means:

• a traditional letter
• an email

In this sentence, if the context is computers or office work, many learners will naturally understand it as email.

If someone wanted to be more explicit about email, they might say:

• к электронному письму = to the email

But к письму is perfectly normal when the context already makes it clear.

Yes, you very often can.

Both verbs can mean attach, but there is a slight difference in feel:

• приложить often means to add / enclose / attach
• прикрепить often feels a bit more like fasten / attach firmly, and is also very common for files in digital contexts

So both of these are natural:

• Пожалуйста, приложите файл к письму.
• Пожалуйста, прикрепите файл к письму.

In computer-related Russian, прикрепить файл is extremely common.
Приложить файл is also correct and natural, especially in more formal or official language.

Russian word order is more flexible than English word order, so yes, it can be changed.

The given version is the most neutral and natural:

• Пожалуйста, приложите файл к письму.

But you may also hear:

• Приложите, пожалуйста, файл к письму.
• Файл, пожалуйста, приложите к письму.
  (more marked; stronger focus on file)

Putting пожалуйста in different places is common. The two most neutral options are:

• Пожалуйста, приложите файл к письму.
• Приложите, пожалуйста, файл к письму.

Приложить is perfective.

That matters because perfective verbs in the imperative often focus on completing a single action:

• приложите = attach it / make sure it gets attached

If you used an imperfective verb in an imperative, the meaning could shift toward process, repetition, or a different nuance. In practical everyday language, when asking someone to attach one file once, the perfective приложите is exactly what you would expect.

A helpful approximate pronunciation is:

• письму ≈ pees-MOO

A few details:

• the stress is on the last syllable: письмУ
• the soft sign ь softens the с
• the final -у sounds like oo in boot

So the whole sentence is approximately:

пажа́луйста, прило́жите файл к письму́

That is only an approximation, but it helps with the main stress pattern.