A missão é importante.

A is the feminine singular definite article, meaning the.

It appears here because missão is a feminine singular noun:

• a missão = the mission

In Portuguese, articles are used very often, and they usually have to match the noun’s gender and number.

In Portuguese, every noun has a grammatical gender, usually masculine or feminine. Missão is a feminine noun, so it takes feminine words with it:

• a missão
• a missão importante

Many nouns ending in -ão are masculine, but not all of them. Gender has to be learned word by word in many cases. So even though -ão often looks masculine to learners, missão is definitely feminine.

The ending -ão is a very common Brazilian Portuguese ending, and it is nasal.

It does not sound like English own.
A rough approximation is something like owng said through the nose, but that is still only approximate.

So missão is roughly pronounced:

• mee-SOWN (with a nasal ending)

A more careful breakdown:

• mi- sounds like mee
• -ssão has stress and ends in a nasal ão

The ss shows that the s sound is a strong s, like in English see.

If it were written with just one s between vowels, it would usually sound like a z in Portuguese. Compare:

• missão → strong s
• a single s between vowels often sounds like z

So ss helps keep the pronunciation as mi-SÃO, not mi-ZÃO.

É is the verb ser in the he/she/it is form, and the accent mark is part of its spelling.

So:

• é = is

This accent matters because e without an accent is usually the word and:

• e = and
• é = is

So the accent helps distinguish two very common words.

Portuguese has two main verbs for to be: ser and estar.

Here, é from ser is used because the sentence describes an essential or general characteristic:

• A missão é importante. = The mission is important.

Use ser for things seen as inherent, defining, or generally true.

Estar is more often used for temporary states or conditions. In many contexts, está importante would sound unusual or would suggest a temporary/changing state in a special context.

So for a normal statement like this, é is the natural choice.

In Portuguese, adjectives often come after the noun:

• a missão importante

That is a very normal word order.

In this sentence, though, importante comes after é because it is part of the predicate:

• A missão = subject
• é importante = what is being said about the subject

So this is not just noun + adjective inside one noun phrase; it is a full statement:

• The mission is important

Some Portuguese adjectives change form for gender:

• bonito / bonita
• cansado / cansada

But importante is an adjective with the same form for masculine and feminine singular:

• o plano importante
• a missão importante

It still agrees in number, though:

• singular: importante
• plural: importantes

So:

• A missão é importante.
• As missões são importantes.

Usually, no—not in a normal neutral sentence like this.

In standard Portuguese, A missão é importante is the natural full sentence. Leaving out the article would sound incomplete or unnatural in most contexts.

You might see article omission in:

• titles
• headlines
• notes
• very compressed style

But for everyday speech and normal writing, use a missão.

A rough pronunciation guide is:

ah mee-SOWN eh eem-por-TAHN-chee

A few notes:

• A is usually a light ah
• missão is stressed on the last syllable
• é is like eh
• importante in Brazil is often pronounced roughly eem-por-TAHN-chee
• the an sound in -tan- is nasal
• the final te often sounds like chee in many Brazilian accents

A more connected natural rhythm would sound something like:

a missão é importante

with the main stress on:

• são
• tan

In the plural, everything that can change should agree:

• A missão é importante.
• As missões são importantes.

Changes:

• a → as
• missão → missões
• é → são
• importante → importantes

This is a good example of agreement in Portuguese:

• article agrees with noun
• verb agrees with subject
• adjective agrees in number

Yes, but it means something different.

• A missão é importante. = The mission is important.
• Uma missão é importante. = A mission is important.

So a points to a specific or understood mission, while uma introduces an indefinite one.

In context, the choice depends on whether you mean:

• a particular mission already known to the listener, or
• one mission in a more general or indefinite sense

You can change the word order, but it changes the emphasis.

The neutral, standard order is:

• A missão é importante.

A more marked or emphatic order is:

• Importante é a missão.

That sounds more literary, contrastive, or dramatic—something like:

• What is important is the mission

So for most everyday situations, stick with A missão é importante.