Enviei a participação ontem.

Because Portuguese often drops the subject pronoun when the verb ending already makes the subject clear.

• Enviei = I sent
• The ending -ei shows it is eu (I), in the past.

So:

• Enviei a participação ontem = I sent the participation/submission yesterday
• Eu enviei a participação ontem = also correct, but eu is usually only added for emphasis, contrast, or clarity.

Enviei is the 1st person singular of the pretérito perfeito simples of enviar.

In English, this usually corresponds to the simple past:

• enviei = I sent

This tense is used for a completed action in the past, especially when it happened at a definite time, such as ontem (yesterday).

The verb is enviar (to send).

It is an -ar verb, so in the pretérito perfeito:

• eu enviei
• tu enviaste
• ele/ela enviou
• nós enviámos (European Portuguese spelling)
• vós enviastes
• eles/elas enviaram

So enviei is the eu form.

Here, a is the definite article meaning the.

• a participação = the participation / the submission / the entry

Portuguese uses articles more often than English does in some contexts. So even if English might sometimes say I sent my entry or I sent an entry, Portuguese may naturally use a participação, depending on context.

Not always. Participação can mean different things depending on context.

Common possibilities include:

• participation
• entry (for a contest or event)
• submission
• report
• notification

So in some situations, Enviei a participação ontem may sound more natural in English as:

• I sent the entry yesterday
• I submitted it yesterday
• I sent the participation form yesterday

The exact meaning depends on the situation.

Because a participação is the direct object of the verb enviar.

• enviar algo = to send something

So:

• Enviei a participação = I sent the participation/submission

No extra preposition is needed before a normal direct object.

Putting ontem at the end is a very natural and neutral word order in Portuguese.

• Enviei a participação ontem. = very normal
• Ontem enviei a participação. = also correct, with more focus on yesterday

Portuguese word order is fairly flexible, but the version with ontem at the end is especially common in everyday speech.

Yes, absolutely.

• Enviei a participação ontem = neutral, natural
• Eu enviei a participação ontem = also correct

Adding eu may sound:

• more emphatic
• contrastive
• clearer if needed

For example:

• Eu enviei a participação ontem, mas ele ainda não enviou a dele.
  I sent the submission yesterday, but he still hasn’t sent his.

In most cases, it is more like I sent.

Since the sentence includes ontem (yesterday), English strongly prefers the simple past:

• I sent the submission yesterday

Portuguese pretérito perfeito simples often covers what English expresses with the simple past. With a finished time expression like yesterday, I have sent yesterday would not be natural English.

In European Portuguese, the pronunciation is roughly:

• Enviei → something like en-vee-AY
• a participação → uh par-tee-see-puh-SOWN
• ontem → roughly ON-teng or ON-tayn depending on accent

A few useful notes:

• The final -ei in enviei sounds like ay
• ção in participação has a nasal sound, roughly like sown but nasal
• Unstressed a often sounds like a reduced uh in European Portuguese

Sometimes yes, but not always with exactly the same nuance.

• enviar = to send
• mandar can also mean to send, but it is broader and can also mean to order, to tell, or to have something sent

In many everyday contexts:

• Enviei o email ontem.
• Mandei o email ontem.

Both can work.

But enviar often sounds a bit more directly equivalent to send, while mandar can feel slightly more informal or broader in use. For a learner, enviar is the safer match here.

Yes, depending on what is actually being sent.

For example:

• Enviei o formulário ontem. = I sent the form yesterday.
• Enviei a inscrição ontem. = I sent the registration/application yesterday.
• Enviei a candidatura ontem. = I sent the application yesterday.
• Enviei o email ontem. = I sent the email yesterday.

So if participação feels unusual, that is often because it is tied to a specific context, such as a contest, event, complaint, report, or official submission.