O jejum não é necessário.

O is the masculine singular definite article, meaning the.

In Portuguese, it is very common to use the definite article before a noun even when English would not. So O jejum não é necessário literally looks like The fasting is not necessary, but the natural English meaning is simply Fasting is not necessary or A fast is not necessary, depending on context.

Here, jejum is a masculine singular noun, so the article is o.

Jejum is a noun meaning fasting or a fast.

It is commonly used in medical, religious, and everyday contexts:

• fazer jejum = to fast
• estar em jejum = to be fasting / to have an empty stomach
• exame em jejum = a test done after fasting

In this sentence, jejum refers to the act or state of fasting.

In Portuguese, the normal way to make a sentence negative is to put não directly before the verb.

So:

• é necessário = is necessary
• não é necessário = is not necessary

This is very standard word order in Portuguese.

It uses é because the sentence is expressing a general fact or classification, not a temporary condition.

• ser is used for identity, definition, classification, and general statements.
• estar is usually used for temporary states or conditions.

So:

• O jejum não é necessário = Fasting is not necessary.
  This sounds like a general statement.

If you said está necessário, that would sound unnatural here.

É is the third-person singular present tense of the verb ser.

So:

• eu sou = I am
• você é / ele é / ela é = you are / he is / she is

Here the subject is o jejum, which is singular, so the verb is é.

Because necessário agrees with jejum, and jejum is a masculine singular noun.

In Portuguese, adjectives usually agree in gender and number with the noun they describe:

• o jejum necessário = the necessary fast
• a consulta necessária = the necessary appointment

Since jejum is masculine singular, the adjective is also masculine singular: necessário.

You usually learn the gender of a noun together with its article:

• o jejum

That tells you it is masculine.

There is not always a simple rule based on meaning, so it is best to memorize nouns with their articles:

• o jejum
• a dieta
• o exame

A simple approximation is:

oo zhe-ZHOONG now eh neh-seh-SAH-ryoo

A few key points:

• j in jejum sounds like the s in measure
• não has a nasal sound, not a clear English now
• é is an open vowel, like eh
• necessário is stressed on sá

A more Portuguese-like pronunciation would be roughly: u zhe-ZHUM nãw é neh-seh-SAH-ryu

The final m in jejum nasalizes the vowel; it is not pronounced like a strong English m.

The accent marks help show pronunciation and sometimes distinguish words.

• não: the tilde (ã) shows nasal pronunciation
• é: the acute accent shows stress and also distinguishes it from e meaning and
• necessário: the acute accent marks the stressed syllable: sá

So the stress is:

• não
• é
• ne-ces-sá-rio

Yes, you may hear or read Jejum não é necessário, especially in headlines, signs, instructions, or more concise styles.

But O jejum não é necessário is very natural and complete in normal speech and writing.

So:

• O jejum não é necessário = very standard
• Jejum não é necessário = also possible, often a bit more concise or impersonal

Yes. Some natural alternatives are:

• Não é necessário fazer jejum. = It is not necessary to fast.
• Você não precisa fazer jejum. = You do not need to fast.
• Jejum não é necessário. = Fasting is not necessary.

All of these are natural, but they are slightly different in structure:

• O jejum não é necessário focuses on jejum as the subject
• Não é necessário fazer jejum focuses on the action to fast
• Você não precisa fazer jejum addresses the listener directly

It is an adjective, and like in English, it can appear after the verb ser.

So in:

• O jejum não é necessário

necessário is still an adjective describing jejum, even though it comes after the verb.

This is completely normal in Portuguese:

• O exame é importante.
• A consulta é urgente.
• O jejum não é necessário.

It sounds neutral and very natural. It could easily appear in:

• medical instructions
• hospital or lab information
• general explanations
• everyday conversation

It is neither especially formal nor especially casual. It is a straightforward standard sentence.