A fonte da informação é segura.

Because da is the normal contraction of de + a in Portuguese.

• de = of / from
• a = the (feminine singular)
• de + a = da

So:

• a fonte da informação = the source of the information

This kind of contraction is very common in Portuguese and is required in standard language.

Other common examples:

• do = de + o
• na = em + a
• no = em + o

In Portuguese, definite articles are used more often than in English. Here, A fonte means the source.

So:

• A fonte da informação é segura. = The source of the information is reliable/secure.

Leaving out the article here would sound unnatural in normal Portuguese.

Fonte literally can mean several things, depending on context:

• source
• spring
• fountain
• sometimes font in certain technical contexts

In this sentence, fonte means source, especially the origin of some information.

So a fonte da informação means:

• the source of the information
• in natural English, often just the information source

Because segura is agreeing with fonte, which is a feminine singular noun.

• a fonte → feminine singular
• adjective must match → segura

Compare:

• A fonte é segura. = feminine singular
• O sistema é seguro. = masculine singular
• As fontes são seguras. = feminine plural
• Os dados são seguros. = masculine plural

This kind of adjective agreement is a basic feature of Portuguese.

In this sentence, segura most naturally means reliable, trustworthy, or dependable.

Even though seguro/segura often means safe/secure, with source of information the intended idea is usually:

• trustworthy
• reliable
• credible

So A fonte da informação é segura would usually be understood as:

• The source of the information is reliable.

É is the 3rd person singular of the verb ser in the present tense.

• ser = to be
• é = is

Since the subject is A fonte da informação (The source of the information), singular, the verb must also be singular:

• A fonte da informação é segura.
• The source of the information is reliable.

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

Here, ser is used because the sentence describes a characteristic of the source: its reliability or trustworthiness.

• ser = identity, essential quality, classification, general characteristic
• estar = temporary state, condition, location in many cases

So:

• A fonte da informação é segura.
  = The source is reliable / trustworthy as a characteristic.

If you used estar segura, it would usually suggest a temporary condition or a different meaning depending on context, and it would sound less natural here.

Yes, but the meaning changes slightly.

• A fonte da informação = the source of the information
  This refers to a specific piece of information.

• A fonte de informação = the information source or a source of information
  This is more general.

So:

• A fonte da informação é segura.
  = the source behind this particular information is reliable

• A fonte de informação é segura.
  = the information source is reliable / it is a reliable source of information

This is a very useful distinction:

• de + article often points to something specific
• de + noun without article is often more general

Yes, in this sentence the word order is very close to English:

• A fonte = the source
• da informação = of the information
• é = is
• segura = reliable / secure

So the structure is:

subject + linking verb + adjective

That said, Portuguese can sometimes be more flexible than English, but this is the most neutral and natural order here.

In European Portuguese, informação ends in -ão, which is a nasal sound.

Important points:

• the stress is on the final syllable: informaÇÃO
• ão is nasal, not pronounced like English ow
• the ç gives an s sound

A rough guide for English speakers:

• informação ≈ in-for-ma-SOWN
  but with a nasal ending, not a full English own

Also, the accent mark in -ção helps show the stressed syllable.

In European Portuguese, the pronunciation is more reduced than many learners expect, especially in unstressed syllables.

A rough learner-friendly guide:

• A fonte da informação é segura.

Approximate European Portuguese pronunciation:

• uh FON-tuh duh een-for-ma-SOUN eh suh-GOO-ruh

A few notes:

• A often sounds like a very weak uh
• da is also reduced
• informação has the stressed nasal ending -ção
• é is clearly pronounced, like eh
• segura has the stress on gu: se-GU-ra

This is only an approximation, but it helps with the rhythm.

The accent mark in informação helps indicate stress.

• informação is stressed on the final syllable: informaÇÃO

Without the accent, the learner would not know as easily where the stress belongs.

Also:

• ç is pronounced like s
• ão is a nasal ending very common in Portuguese

So the spelling gives you two important pronunciation clues:

• ç = s sound
• ã/ão = nasal sound

It is neutral and perfectly natural in both spoken and written Portuguese.

It could appear in:

• conversation
• journalism
• academic writing
• workplace language

It is not slangy, and it is not especially formal either. It is just standard Portuguese.

Yes. Although segura works, depending on context Portuguese speakers might also use:

• fiável = reliable
• confiável = trustworthy, reliable
• credível = credible

In European Portuguese, fiável is especially common.

So you might also hear:

• A fonte da informação é fiável.

That often sounds very natural when talking about the reliability of a source.

Still, segura is understandable and correct, especially if the intended meaning is trustworthy/secure.