Learn Hardcore Portuguese (Portugal): Enviei o relatório em anexo, mas a professora disse que não abria. - I sent the report as an attachment, but the teacher said it wouldn’t open.

As an adjective, anexo/anexa/anexos/anexas agrees with the noun: Segue anexo o relatório. / Seguem anexos os documentos.
Anexado is the past participle (“attached”), acceptable but more bureaucratic/technical in tone; in everyday emails, em anexo or adjectival anexo is more idiomatic.
Don’t confuse em anexo (attached) with no anexo (“in the attachment/appendix”), which refers to content inside the attachment or an appendix section.

Can I change the word order of em anexo?

Yes. All of these are fine in European Portuguese:

Enviei o relatório em anexo.
Enviei em anexo o relatório.
Em anexo, envio o relatório. They differ only in emphasis and style, not meaning.

Is the definite article o in o relatório required?

In Portuguese, definite articles are used more often than in English. If both speaker and listener know which report is meant, o relatório is natural. Um relatório would suggest “a/one report” (non-specific) and wouldn’t fit as well here.

Why is the imperfect abria used after disse que instead of abriu?

With reported speech, Portuguese often “backshifts” the tense to the imperfect to show simultaneity with a past reference point and to give an imperfective feel:

Direct speech: O relatório não abre.
Reported: A professora disse que o relatório não abria. Here, não abria conveys “wouldn’t open/wasn’t opening” at that time, which matches English “wouldn’t open.”

Could I say disse que não abriu instead? What’s the difference?

You can, but it slightly changes the nuance:

…disse que não abria = “said it wouldn’t open/wasn’t opening” (state/attempts over a time frame; common in reported complaints).
…disse que não abriu = “said it didn’t open” (a single, completed event). If the teacher described repeated failed attempts or a general inability at that moment, não abria is more idiomatic.

Who is the subject of abria? Is it the teacher or the report?

It’s the report (understood from context). Abrir can be intransitive for things that open (files, doors, etc.): O relatório não abria = “The report wouldn’t open.” If you want to avoid any ambiguity, you can make it explicit: …disse que o relatório não abria or rephrase as …disse que não conseguia abrir o relatório.

Why not the subjunctive, like disse que não abrisse?

Because this is a statement of fact in reported speech, not a command or a hypothetical. Use the subjunctive when reporting orders/requests or expressing non-factuality:

Reported order: Ela disse para eu não abrir o relatório / Ela disse que eu não abrisse o relatório.
Reported fact/complaint: …disse que não abria.

If I want to say “She told me,” how do I add “me” in European Portuguese?

Add the clitic -me to the verb: …mas a professora disse-me que…. In EP, clitics often attach to the verb in affirmative statements. You could also say …a professora disse-me que não abria.

How would I say “she couldn’t open it” more explicitly?

Several natural options:

…disse que não conseguia abrir o relatório.
With a pronoun: …disse que não o conseguia abrir or …disse que não conseguia abri-lo.
Colloquial: …disse que não dava para abrir.

Can I use the present perfect, like tenho enviado?

Not here. Portuguese tenho enviado means “I have been sending (repeatedly/over a period),” not a single completed action. For a one-time action in the past, use enviei. To highlight that sending happened before another past reference, you could use past perfect: tinha enviado.

Are there useful synonyms or register differences for enviar?

Enviar is neutral/formal.
Mandar is common and slightly more informal: Mandei o relatório em anexo.
Remeter is more formal/bureaucratic: Remeti o relatório em anexo.

Any European vs Brazilian Portuguese differences I should note?

EP prefers disse que; in Brazil you’ll also hear falou que (informal).
EP uses clitic positions like disse-me; in BR you’ll usually hear me disse.
Vocabulary: EP often says ficheiro (file), BR arquivo; both use anexo for email attachment. The sentence as given is fully natural in European Portuguese.

How would I replace the noun with a pronoun: “I sent it attached”?

In EP: Enviei-o em anexo. If you also include an indirect object (“I sent it to her”), you can say Enviei-lho em anexo (to her = lhe + it = o → lho). That last form is correct but quite formal/rare in everyday speech; most people would prefer something like Enviei o relatório à professora, em anexo.

Is the comma before mas correct?

Yes. In Portuguese, you normally place a comma before mas (“but”): …, mas … It separates contrasting clauses.

Is relatório the right word for a student assignment?

Often yes, if it’s a report-style assignment. For coursework in general, trabalho is very common; use relatório when it’s specifically a report. Example: Enviei o trabalho em anexo… if you mean “assignment.”