Estoy absolutamente seguro.

Because seguro here describes a state or condition: being sure about something at this moment. In Spanish, that usually takes estar, not ser.

• Estoy seguro = I am sure
• Soy seguro would usually not work for this meaning

A useful contrast is:

• estar seguro de algo = to be sure about something
• ser seguro = to be safe / secure, depending on context

So in Estoy absolutamente seguro, estar is the correct verb.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• Estoy already means I am
• So Yo estoy absolutamente seguro is possible, but usually unnecessary

You would add yo only for emphasis, contrast, or clarity, for example:

• Yo estoy absolutamente seguro, pero él no.
  I’m absolutely sure, but he isn’t.

Absolutamente is an adverb meaning absolutely. It intensifies seguro.

So:

• Estoy seguro = I’m sure
• Estoy absolutamente seguro = I’m absolutely sure

It works much like English absolutely, adding stronger emphasis.

Because absolutamente modifies the adjective seguro, and in Spanish, adverbs commonly come before adjectives.

So:

• absolutamente seguro = absolutely sure

This is the most natural order. Spanish generally does not say seguro absolutamente in this kind of sentence.

Yes. Seguro is an adjective, so it must agree in gender and number with the person or people being described.

Examples:

• Estoy absolutamente seguro = said by a man
• Estoy absolutamente segura = said by a woman
• Estamos absolutamente seguros = we are absolutely sure, masculine or mixed group
• Estamos absolutamente seguras = we are absolutely sure, all women

So if a female speaker is talking about herself, she would normally say Estoy absolutamente segura.

Not in this sentence by itself. Estoy absolutamente seguro is a complete sentence meaning I’m absolutely sure.

But if you want to say what you are sure about, then you often add de:

• Estoy seguro de eso. = I’m sure of that.
• Estoy seguro de que vendrá. = I’m sure that he/she will come.

So:

• without complement: Estoy absolutamente seguro.
• with complement: Estoy absolutamente seguro de...

Yes, but it is not exactly the same strength.

• muy seguro = very sure
• absolutamente seguro = absolutely sure

Absolutamente sounds stronger and more emphatic. It suggests total certainty.

Other common intensifiers include:

• Estoy completamente seguro = I’m completely sure
• Estoy totalmente seguro = I’m totally sure

In standard Spanish from Spain, estoy is pronounced roughly like es-TOY, with the stress on the second syllable.

A rough guide:

• es like ess
• toy rhymes approximately with English boy

So: es-TOY

The y at the end sounds like the vowel sound in boy, not like the English consonant y in yes.

It can mean several related things depending on context.

Common meanings include:

• sure / certain
  Estoy seguro. = I’m sure.
• safe / secure
  Es un lugar seguro. = It’s a safe place.
• insurance as a noun
  un seguro = an insurance policy / insurance

So in Estoy absolutamente seguro, it clearly means sure / certain.

Yes. Estoy absolutamente seguro is neutral and natural in both formal and informal Spanish.

It can fit many situations, for example:

• casual conversation
• professional discussion
• argument or debate
• written Spanish

If you want to sound a little softer or less forceful, you might choose:

• Creo que sí. = I think so.
• Estoy bastante seguro. = I’m quite sure.

But Estoy absolutamente seguro is perfectly correct and natural.

Yes. Estoy seguro is the basic version and is extremely common.

Compare:

• Estoy seguro. = I’m sure.
• Estoy absolutamente seguro. = I’m absolutely sure.

Adding absolutamente just makes the statement stronger.

It is understood everywhere in the Spanish-speaking world. Nothing in Estoy absolutamente seguro is exclusive to Spain.

Speakers in Spain and Latin America would all understand it, and many would use it naturally. The only thing that might vary a little is pronunciation or how often people choose absolutamente versus other intensifiers like totalmente or completamente.