O formulário deve ser preenchido antes da reunião.

Portuguese often uses the definite article more than English does. O formulário means the form.

In a sentence like this, using o sounds natural because it refers to a specific form that people already know about. Leaving the article out would usually sound less natural here.

Also, formulário is a masculine singular noun, so it takes o:

• o formulário
• o documento
• o papel

If it were feminine, you would use a instead:

• a reunião
• a ficha

Deve comes from the verb dever. In this sentence, it expresses obligation or requirement.

Depending on context, it can be understood as:

• must
• should
• is supposed to
• needs to

In formal instructions like this one, deve often sounds like must / should be.

Examples:

• Você deve assinar aqui. = You must / should sign here.
• O formulário deve ser enviado hoje. = The form must / should be sent today.

So the exact English translation can vary, but the Portuguese structure stays the same.

Because Portuguese needs the infinitive ser to form this kind of passive structure.

The pattern is:

dever + ser + past participle

So:

• deve ser preenchido = must be filled out
• deve ser enviado = must be sent
• deve ser assinado = must be signed

You cannot say deve preenchido because preenchido is a past participle, and it needs the auxiliary ser in this passive construction.

Preenchido is the past participle of preencher, which means to fill in / to fill out.

In the sentence, it is part of the passive voice:

• ser preenchido = to be filled out

Compare:

• Active: Você deve preencher o formulário. = You must fill out the form.
• Passive: O formulário deve ser preenchido. = The form must be filled out.

So preenchido is used because the sentence focuses on the form, not on who fills it out.

Because it agrees with formulário, which is masculine singular.

In Portuguese, past participles used with ser in passive voice usually agree in gender and number with the noun.

Here:

• o formulário = masculine singular
• so: preenchido

Compare:

• A ficha deve ser preenchida.
  feminine singular
• Os formulários devem ser preenchidos.
  masculine plural
• As fichas devem ser preenchidas.
  feminine plural

Antes de means before.

Here it introduces the thing that happens later:

• antes da reunião = before the meeting

This is a very common expression:

• antes do almoço = before lunch
• antes da aula = before class
• antes de viajar = before traveling

Notice that:

• before a noun: antes de + noun
• before a verb: antes de + infinitive

Examples:

• antes da reunião = before the meeting
• antes de sair = before leaving

Because de + a contracts to da in Portuguese.

So:

• de + a reunião → da reunião

This kind of contraction is extremely common:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

Examples:

• antes do trabalho
• depois da aula
• o início da reunião

So antes da reunião is simply the normal contracted form of antes de a reunião.

Noun gender in Portuguese is partly predictable, but often it just has to be learned with the word.

Reunião is a feminine noun, so it takes:

• a reunião
• da reunião
• uma reunião

Many nouns ending in -ão are masculine, but not all of them. So it is important to learn nouns together with their article:

• o formulário
• a reunião

That helps you remember agreement later.

Yes, this is a passive sentence.

Passive:

• O formulário deve ser preenchido antes da reunião.

A natural active version would be:

• Você deve preencher o formulário antes da reunião.
• Os participantes devem preencher o formulário antes da reunião.

The passive is common in formal instructions because it focuses on the task itself, not on the person doing it.

English does this too:

• The form must be filled out...

Sometimes, but preenchido is usually the most natural word for forms.

For forms, fields, applications, and paperwork, Portuguese commonly uses preencher:

• preencher um formulário
• preencher um cadastro
• preencher os dados

Completar usually means to complete or to finish, and in some contexts it can work, but it is less standard for this exact idea.

So for a learner, the safest choice is:

• preencher um formulário

The verbs are:

• deve → dictionary form: dever
• ser → dictionary form: ser
• preenchido → comes from preencher

So the full verbal idea is based on:

• dever = to have to / must / should
• ser = to be
• preencher = to fill out

This helps you recognize other forms:

• devem ser preenchidos
• deverá ser preenchido
• foi preenchido

Normally, no. Antes da reunião is the natural form.

Portuguese usually keeps the article with a noun like reunião in this kind of sentence:

• antes da reunião
• depois da aula
• durante o evento

Leaving the article out would sound unusual in standard Portuguese here.

You would make the noun, verb, and participle agree:

• Os formulários devem ser preenchidos antes da reunião.

Changes:

• o → os
• formulário → formulários
• deve → devem
• preenchido → preenchidos

If the meeting were also plural, that part would change too:

• antes das reuniões

Yes, it sounds fairly formal and very natural in written instructions, official notices, workplace communication, and administrative contexts.

It fits well in:

• emails
• signs
• company procedures
• event instructions
• government or school documents

A less formal active version might be:

• Você precisa preencher o formulário antes da reunião.
• Preencha o formulário antes da reunião.

But the original sentence is perfectly normal and common in formal Brazilian Portuguese.