Eu não sei o que fazer hoje.

In this sentence, o que works together as “what” in an embedded clause:

• Eu não sei o que fazer hoje.
  I don’t know what to do today.

In European Portuguese:

• o que = what (as a pronoun, “the thing that”)
• que alone is usually that/which/that… as a conjunction, or part of other structures.

Here, o is a definite article (the) and que is a relative/interrogative pronoun; together they form something like “the thing that”, which in natural English is just “what”:

• (Eu não sei) o [que fazer hoje]
  → literally: “I do not know the [what to do today].”

If you said Eu não sei que fazer hoje, it would sound old‑fashioned / literary and is not what learners should use in everyday speech.

Here fazer is in the infinitive (to do) because it is part of the object of sei:

• Eu não sei [o que fazer hoje].
  → “I don’t know [what to do today].”

After saber (to know) in this type of structure, European Portuguese often uses an infinitive to express what action you don’t know how / whether / when to do:

• Não sei o que dizer. – I don’t know what to say.
• Não sei onde ir. – I don’t know where to go.
• Não sei que livro comprar. – I don’t know which book to buy.

Using a conjugated form like Eu não sei o que faço hoje means something different: more like

• “I don’t know what it is that I do today / what I’m doing today”
  (commenting on your schedule / behaviour), not simply “I don’t know what to do” as a decision.

Yes.

Subject pronouns are often omitted in Portuguese when the verb ending already shows the subject:

• (Eu) não sei o que fazer hoje.

In speech, especially in European Portuguese, you will very often hear:

• Não sei o que fazer hoje.

Keeping Eu can add a bit of emphasis on I:

• Eu não sei o que fazer hoje.
  → I don’t know what to do today (maybe others do).

Não normally goes directly before the conjugated verb it negates:

• Eu não sei o que fazer hoje. – correct
  (não
  • sei)

Eu sei não o que fazer hoje is wrong in standard Portuguese.

Basic pattern:

• não + [conjugated verb] + (rest of sentence)

Examples:

• Eu não quero sair. – I don’t want to go out.
• Ela não gosta de café. – She doesn’t like coffee.
• Nós não podemos ajudar. – We cannot help.

Yes. Time expressions like hoje are flexible. All of these are possible and natural, with slight differences in emphasis:

1. Eu não sei o que fazer hoje.
   → Neutral: “I don’t know what to do today.”

2. Hoje não sei o que fazer.
   → Emphasis on today: “Today I don’t know what to do.”

3. Eu hoje não sei o que fazer.
   → Also emphasizes hoje a bit, common in European Portuguese.

4. Não sei o que fazer hoje.
   → Quite common and natural, subject Eu omitted.

What you should not do is break up o que fazer:

• ✗ Eu não sei o que hoje fazer. – sounds wrong / very unnatural.

Use saber here, not conhecer.

Rough guide:

• saber = to know information, facts, how to do something

  • Eu não sei o que fazer hoje. – I don’t know what to do today.
  • Ela sabe falar português. – She knows how to speak Portuguese.

• conhecer = to be familiar with / to know a person, place, or thing

  • Conheço Lisboa. – I know Lisbon / I’m familiar with Lisbon.
  • Não conheço esse livro. – I don’t know that book.

So:

• ✗ Eu não conheço o que fazer hoje. – incorrect / unnatural.
  You must say:
• ✓ Eu não sei o que fazer hoje.

Saber is irregular in the present tense. The 1st person singular (eu) form is:

• eu sei – I know

Key present forms of saber:

• eu sei – I know
• tu sabes – you know (singular, informal)
• ele / ela sabe – he/she knows
• nós sabemos – we know
• vocês sabem – you (plural) know
• eles / elas sabem – they know

So the correct form in the sentence is:

• Eu não sei o que fazer hoje.

The sentence contains an indirect question, not a direct one.

• O que fazer hoje? – What to do today? → direct question (would take a question mark).
• Eu não sei o que fazer hoje. – I don’t know what to do today. → indirect question (embedded inside a statement).

In English we also do this:

• What should I do today? – direct question.
• I don’t know what I should do today. – indirect question, no question mark.

Portuguese works the same way: the whole sentence is a statement, so it ends with a period.

All are possible, but they feel a bit different:

1. Eu não sei o que fazer hoje.

   • Very common and neutral.
   • Focuses on the available options / what action to take.

2. Eu não sei o que vou fazer hoje.

   • Literally: I don’t know what I am going to do today.
   • Feels more like you are unsure about your plans or schedule, not necessarily searching for ideas.

3. Eu não sei o que hei de fazer hoje.

   • More typical of European Portuguese; sounds quite natural, maybe a bit more “thoughtful” or slightly old‑fashioned.
   • Often used when you’re undecided or worried:
     
     • Não sei o que hei de fazer. – “I don’t know what I should do.”

All three can be correct, but Eu não sei o que fazer hoje is the simplest and most general for “I don’t know what to do today.”

Yes, absolutely.

Dropping Eu is very common and sounds natural:

• Não sei o que fazer hoje.

In a casual context, this may be even more frequent than the full version with Eu. The meaning is identical; the verb ending -ei in sei already shows the subject.

In careful European Portuguese, roughly:

• Eu não sei o que fazer hoje
  IPA (approx.): [ew nɐ̃w ˈsɐj u kɨ fɐˈzeɾ ˈoʒ(ɨ)]

Some key points:

• Eu – often sounds like [ew], sometimes very short, almost [e].
• não – nasal vowel [ɐ̃w]; the ão is nasal and ends in a light w sound.
• sei – like “say”, but a bit tenser: [sɐj].
• o – short [u] or [o] depending on accent; often quite reduced.
• que – usually [kɨ] in European Portuguese (very reduced vowel).
• fazer – [fɐˈzeɾ], with the final r pronounced (but weaker than in English).
• hoje – [ˈoʒ(ɨ)], the final e is often almost silent or very reduced.

In fast speech, you’ll hear linking and reduction:

• Sounds a bit like: [ew nɐ̃wˈsɐju kɨ fɐˈzeɾˈoʒ]
  (everything flowing together).

No.

In Portuguese, subject pronouns like eu, tu, ele, ela, etc. are not capitalised in the middle of a sentence, unless they start the sentence:

• Eu não sei o que fazer hoje. – at the start, Eu is capitalised.
• Disse que eu não sei o que fazer hoje. – in the middle, eu is lower‑case.

So you should write:

• Eu não sei o que fazer hoje.
  or (without the pronoun)
• Não sei o que fazer hoje.