Atașez nu numai factura, ci și formularul de retur.

Atașez means I attach.

It is:

• present tense
• 1st person singular
• from the verb a atașa = to attach

Romanian often leaves out the subject pronoun, so Atașez by itself already means I attach. You do not need eu unless you want emphasis.

Romanian is a pro-drop language, which means the subject pronoun is often omitted because the verb ending already shows who is doing the action.

So:

• Atașez = I attach
• Eu atașez = I attach, but with extra emphasis, contrast, or clarity

In normal writing, especially in emails or formal messages, leaving out eu is completely natural.

This is a very common Romanian structure meaning not only ..., but also ...

In your sentence:

• nu numai factura = not only the invoice
• ci și formularul de retur = but also the return form

So the full pattern is:

• nu numai X, ci și Y = not only X, but also Y

A few examples:

• Am cumpărat nu numai pâine, ci și lapte.
  = I bought not only bread, but also milk.
• Ea este nu numai inteligentă, ci și foarte amabilă.
  = She is not only intelligent, but also very kind.

Yes. Nu doar and nu numai are both common ways to say not only.

So these are both natural:

• Atașez nu numai factura, ci și formularul de retur.
• Atașez nu doar factura, ci și formularul de retur.

Nu numai may sound a little more formal or careful in some contexts, but both are correct.

Because Romanian usually uses the definite article when talking about a specific known item.

• factură = an invoice / invoice
• factura = the invoice

In this sentence, the speaker means a specific invoice and a specific return form, so the definite forms are used:

• factura = the invoice
• formularul = the form

Romanian often uses the definite article in places where English might or might not use the, depending on context.

-ul is the masculine/neuter singular definite article attached to the end of the noun.

So:

• formular = form
• formularul = the form

This is one of the big differences from English: Romanian usually puts the at the end of the noun instead of before it.

More examples:

• băiat = boy
• băiatul = the boy
• document = document
• documentul = the document

Because de + noun is a very common Romanian way to express a type, purpose, or relationship between nouns.

So:

• formular de retur = return form
• literally: form of return

This is a standard construction in Romanian. Similar examples:

• bilet de avion = plane ticket
• periuță de dinți = toothbrush
• formular de înscriere = registration form

Using returului would sound unnatural here. De retur is the normal fixed expression.

Retur means return, especially in shopping, shipping, or product-return contexts.

So:

• formular de retur = return form
• politică de retur = return policy

It is a common noun in commercial language and online shopping.

The given word order is natural and clear:

• Atașez nu numai factura, ci și formularul de retur.

Romanian word order is somewhat flexible, but the paired structure nu numai ..., ci și ... should stay together clearly.

You might also see:

• Atașez factura, dar și formularul de retur.

But that changes the nuance a little and is not the same neat not only ... but also ... pattern.

So for this meaning, the original structure is the best choice.

It sounds neutral to formal, and it is very natural in an email or business message.

For example, it fits well in contexts like:

• customer service
• office communication
• shipping or returns
• attaching documents to an email

Atașez is especially common in written communication when sending files or documents.

Yes, in many business contexts you can.

• Atașez = I attach
• Anexez = I enclose / I attach

Both can work when referring to documents.
However:

• atașez is very common for email attachments
• anexez can sound slightly more formal or administrative

So in an email, Atașez nu numai factura, ci și formularul de retur. is perfectly natural.

A simple approximate pronunciation is:

ah-tuh-SHEHZ

A bit more carefully:

• a as in father
• ta with a short uh-like unstressed vowel in normal speech
• ș = sh
• ez sounds roughly like ezh

The letter ș is always pronounced sh, so atașez has that sh sound in the middle.

Here și means also.

So:

• ci = but rather / but
• și = also

Together:

• ci și = but also

This pair works with nu numai to create the full contrast:

• nu numai X, ci și Y = not only X, but also Y

You should usually learn it as a set phrase rather than translating each word separately every time.