A missão parece impossível.

Because missão is a feminine noun in Portuguese, so it takes the feminine singular article a.

• a missão = the mission
• uma missão = a mission

The gender of a noun in Portuguese is grammatical, so it is something you usually have to learn with the noun. In this case, missão is feminine.

Parece is the third-person singular form of the verb parecer, which means to seem or to appear.

Here, the subject is a missão, which is singular, so Portuguese uses:

• A missão parece... = The mission seems...

Other forms of parecer include:

• eu pareço = I seem
• você parece = you seem
• eles parecem = they seem

So parece matches the singular subject a missão.

Because parecer itself links the subject to the description, much like to seem does in English.

In English:

• The mission seems impossible.

In Portuguese:

• A missão parece impossível.

You do not need ser or estar here, because parece is already doing the job of connecting a missão with impossível.

Because impossível is an adjective, and here it describes a missão.

• impossível = impossible
• impossivelmente = impossibly

After parecer, Portuguese normally uses an adjective, just as English does after seem:

• A missão parece impossível.
• The mission seems impossible.

If you used impossivelmente, that would be an adverb, and it would not fit this sentence structure.

In this sentence, impossível already has the same form for both masculine and feminine singular.

So you get:

• o plano parece impossível
• a missão parece impossível

The adjective only changes in the plural:

• os planos parecem impossíveis
• as missões parecem impossíveis

So it does agree with the noun, but in the singular the masculine and feminine forms happen to look the same.

This is the normal and most natural order:

• subject + verb + adjective/complement

So:

• A missão = subject
• parece = verb
• impossível = complement describing the subject

This is very similar to English: The mission seems impossible.

You generally would not say A impossível missão parece in normal speech. Portuguese can sometimes change word order for style or emphasis, but the given sentence is the standard, neutral form.

These are related, but they are not exactly the same.

• parece impossível = seems impossible
  This expresses an impression or appearance.

• é impossível = is impossible
  This sounds stronger and more definite.

• está impossível = is being impossible / is unbearable / is impossible right now
  This often suggests a temporary state or situation, depending on context.

So:

• A missão parece impossível.
  = It looks impossible, maybe from what we can see.

• A missão é impossível.
  = It is impossible.

• A missão está impossível.
  = Less common in this exact context, but it could suggest the mission has become unmanageable or impossibly difficult right now.

The ending -ão is a very common Portuguese noun ending. It is nasal, which is something English speakers often need to practice.

Missão is pronounced roughly like mee-SOWN, but with a nasal ending rather than a fully pronounced n sound.

A few important points:

• the stress falls on the last syllable: mis-SÃO
• -ão is nasal
• you should not pronounce it like plain English ow

Other examples with -ão:

• nação
• decisão
• informação

This ending is extremely common in Portuguese, so it is worth getting used to.

The accent in impossível shows where the stress goes.

It is pronounced roughly im-poh-SEE-vel, with the stress on sí:

• im-pos-sí-vel

Without the accent, a reader might not know the correct stress pattern. Portuguese spelling uses accent marks much more systematically than English does.

So the accent is not optional—it is part of the correct spelling: impossível.

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

Portuguese generally wants an article before many singular countable nouns when you mean the specific thing:

• A missão parece impossível.

If you say Missão parece impossível, it sounds incomplete or unnatural in ordinary usage, unless it is part of a headline, note, or some very specific stylistic context.

So for normal speech and writing, use a missão.

Then the verb and the adjective must both agree in the plural.

Singular:

• A missão parece impossível.

Plural:

• As missões parecem impossíveis.

Changes:

• a → as
• parece → parecem
• impossível → impossíveis

This is a good example of how Portuguese agreement works across the sentence.