A doença é grave.

A is the feminine singular definite article, so it means the.

In A doença é grave, the speaker is referring to the disease in a specific or identifiable sense, or speaking about a disease as the topic under discussion.

A few useful points:

• a = the for feminine singular nouns
• o = the for masculine singular nouns
• doença is feminine, so it takes a

So the structure is:

• A doença = the disease
• é grave = is serious / is severe

Portuguese nouns have grammatical gender, and doença is a feminine noun.

That means it normally goes with feminine words such as:

• a doença = the disease
• uma doença = a disease
• esta doença = this disease

There is not always a logical reason for noun gender, so this is something you usually learn together with the noun. The ending -a often suggests a feminine noun, and doença follows that pattern.

É comes from ser, and está comes from estar.

In this sentence, é grave presents grave as a characteristic or classification of the disease: the disease is serious in nature.

So:

• A doença é grave = the disease is serious / severe

If you used está grave, it could suggest a more temporary or current state, depending on context. With illnesses and medical situations, both verbs can appear, but they do not feel exactly the same.

A simple learner-friendly way to think about it:

• ser = what something is
• estar = how something is at the moment

Here, é grave is the most straightforward and standard choice.

Not all Portuguese adjectives change form for masculine and feminine.

Grave is one of the adjectives that has the same singular form for both genders:

• o problema é grave
• a doença é grave

So even though doença is feminine, grave stays grave.

What does change is the number:

• singular: grave
• plural: graves

For example:

• A doença é grave.
• As doenças são graves.

Yes, it is very close in meaning here.

In Portuguese, grave commonly means:

• serious
• severe

So in a medical context, grave means the illness is serious or severe.

This is a helpful similarity for English speakers, since English grave can also mean serious, although in everyday English serious is more common.

A rough guide is:

uh doo-EN-suh eh GRAH-vuh

A few important details for Portugal Portuguese:

• A is usually reduced, sounding more like a short uh
• the o in doença is unstressed and often sounds like u
• doença has stress on the middle syllable
• the en in doença is nasal
• é sounds like an open eh
• the final -e in grave is not a strong English ee sound; in Portugal it is much weaker, more like a short reduced vowel

A more approximate pronunciation is:

uh du-EN-suh eh GRA-vuh

If you want to sound more natural, pay special attention to the vowel reduction, since that is very typical of European Portuguese.

Because this is a sentence with the verb ser:

• A doença = subject
• é = verb
• grave = predicate adjective

This is the normal structure in Portuguese, just like in English:

• The disease is serious.

So grave comes after the verb because it is describing the subject through é.

This is different from an adjective directly attached to a noun, such as:

• uma doença grave = a serious disease

There, the adjective comes after the noun because it is modifying the noun directly.

Yes, but it means something different.

• A doença é grave = the disease is serious
  This usually refers to a specific disease already known in the conversation.

• Uma doença é grave would sound less natural on its own, because it means a disease is serious, which feels incomplete unless you are making a more general statement or giving an example.

So in an isolated sentence, A doença é grave is much more natural.

The plural is:

As doenças são graves.

Changes:

• A → As
• doença → doenças
• é → são
• grave → graves

So:

• singular: A doença é grave.
• plural: As doenças são graves.

Yes. They are closely related words.

• doença = disease / illness
• doente = ill / sick, or patient / sick person depending on context

Examples:

• A doença é grave. = The disease is serious.
• O doente está cansado. = The patient is tired.

This is a useful word family to learn together.

Not always, but in this sentence it is the natural choice.

You often use the article when talking about:

• a specific disease already known in context
• a topic that has been introduced
• a noun in a general, referential way

So A doença é grave sounds complete and natural.

Without the article, Doença é grave, the sentence would usually sound unnatural as a normal standalone statement.

So for learners, the safest version here is definitely:

A doença é grave.