Está claro que o exame é difícil.

Both Está claro and É claro are possible, but there is a nuance:

• É claro que o exame é difícil.
  – more general, stating a fact: It is clear (as a general truth) that the exam is difficult.

• Está claro que o exame é difícil.
  – slightly more situational: It is (now / at this point / in this context) clear that the exam is difficult.

In everyday speech, though, the difference is often small and both are acceptable. In European Portuguese, you’ll hear both, with É claro que… maybe a bit more common in more formal or written language, and Está claro que… sounding a bit more immediate or contextual.

Grammatically, the subject is the whole clause que o exame é difícil.

You can think of it like English “That the exam is difficult is clear.”
Portuguese usually prefers the “extraposed” version:

• Está claro (main clause)
• que o exame é difícil (subordinate clause functioning as the subject)

So:

• Que o exame é difícil está claro. – is the fully “logical” word order, but sounds quite heavy and is rarely used in everyday speech.
• Está claro que o exame é difícil. – natural, normal sentence.

Claro is an adjective and normally agrees with the noun it refers to:

• uma resposta clara – a clear answer (feminine singular)
• respostas claras – clear answers (feminine plural)

In Está claro que o exame é difícil, the adjective refers to the entire idea que o exame é difícil, which is grammatically treated as masculine singular by default. That’s why you get claro (masculine singular).

You would not say Está clara que o exame é difícil in this structure.
You would only use clara if the adjective clearly referred to a feminine noun:

• A situação está clara. – The situation is clear.
• A resposta está clara. – The answer is clear.

In European Portuguese, definite articles (o, a, os, as) are used more often than in English.

O exame here suggests a specific exam that speaker and listener both know about (for example, the exam you just took, the final exam, etc.).

Compare:

• O exame é difícil. – The exam is difficult (a known, specific one).
• Um exame é difícil. – An exam is difficult (some exam, not specific).
• Exame é difícil. – This sounds very odd or incomplete in Portuguese; you normally need an article.

So o exame is natural and expected in this sentence.

This is the ser vs. estar contrast:

• é difícil (with ser) describes an inherent or typical characteristic of the exam:
  → The exam (by its nature / generally) is difficult.

• está difícil (with estar) describes a temporary or specific state:
  → The exam is being difficult (this time / right now).

In the sentence Está claro que o exame é difícil, the speaker is stating a general characteristic of that exam, not talking only about a temporary impression, so é difícil is the natural choice.

Yes. Está claro que… expresses certainty. In Portuguese, clause-introducing expressions that indicate certainty normally take the indicative:

• Está claro que o exame é difícil.
• É óbvio que ele sabe a resposta.
• É certo que eles vêm amanhã.

You would use the subjunctive after expressions of doubt, possibility, probability, emotion, desire, etc.:

• Duvido que o exame seja difícil.
• É possível que o exame seja difícil.

So with Está claro que…, you keep é difícil in the indicative.

Yes, several expressions are very common:

• É claro que o exame é difícil.
• É óbvio que o exame é difícil. – It’s obvious that…
• É evidente que o exame é difícil. – It’s evident that…
• Não há dúvida de que o exame é difícil. – There’s no doubt that…

They all express certainty, with slightly different emphases (from more neutral é claro to stronger é óbvio / é evidente).

The natural, neutral order is:

• Está claro que o exame é difícil.

Some rearrangements are grammatically possible but sound artificial or emphatic:

• Que o exame é difícil está claro. – grammatically correct, but very heavy and rarely used in normal conversation.
• Está claro que difícil é o exame. – technically possible, but very marked; it strongly emphasizes difícil, sounding almost poetic or theatrical.

For normal speech and writing, stick to:

• Está claro que o exame é difícil.

Approximate IPA for European Portuguese:

• Está claro que o exame é difícil.
  /ʃˈta ˈklaɾu kɨ u ɨˈzɐm(ɨ) ɛ d̪iˈfisil/

Key points:

• Está – es- is reduced; you mostly hear [ʃta].
• claro – stress on cla: [ˈklaɾu].
• que – often very reduced: [kɨ], almost like a very short “k” with a schwa.
• o (article) – often [u].
• exame – stress on -xa-: [ɨˈzɐm(ɨ)]; the final e is a weak [ɨ] or almost silent depending on the speaker.
• difícil – stress on fí: [d̪iˈfisil]; final l is dark/velar [ɫ] in many accents.

Accents in Portuguese mainly mark stress and sometimes vowel quality.

• Está

  • The accent on á shows that the stress is on the last syllable: es-TÁ.
  • Without the accent (esta), it would be the demonstrative esta (this, feminine), stressed on the first syllable: ES-ta.

• difícil

  • The accent on í shows that the stress is on -fí-: di-FÍ-cil.
  • Without it (dificil), the default stress rules would put the stress on the second-to-last syllable (di-FI-cil), which is not correct here.

So accents help you both pronounce and distinguish words correctly.