No estoy del todo seguro de la respuesta.

Because seguro here describes a temporary mental state: how certain someone feels at that moment.

In Spanish, estar is commonly used for states, conditions, and feelings:

• Estoy cansado = I’m tired
• Estoy preocupado = I’m worried
• Estoy seguro = I’m sure

Using ser would sound wrong here because certainty in this sentence is not being treated as a permanent characteristic.

Del todo means completely, entirely, or totally.

So:

• No estoy del todo seguro = I’m not completely sure

It softens the statement a little. Instead of saying I’m not sure, it says I’m not entirely sure.

You will often see del todo in negative sentences:

• No lo entiendo del todo = I don’t fully understand it
• No está del todo mal = It’s not entirely bad

Because the expression is estar seguro de + noun / infinitive / clause.

Examples:

• Estoy seguro de la respuesta = I’m sure of the answer
• Estoy seguro de haberlo visto = I’m sure I saw it
• Estoy seguro de que viene = I’m sure that he’s coming

So de is simply the preposition required by seguro in this structure.

Because in Spanish, seguro normally needs de before the thing you are sure about.

So:

• seguro de algo = sure of something

That is why:

• Estoy seguro de la respuesta not
• Estoy seguro la respuesta

English and Spanish do not always use prepositions in the same way, so this is a pattern worth memorising.

Yes. That is completely natural.

Compare:

• No estoy seguro de la respuesta = I’m not sure of the answer
• No estoy del todo seguro de la respuesta = I’m not completely sure of the answer

The version with del todo sounds a bit more nuanced and slightly less direct.

Yes. Seguro is an adjective, so it changes for gender and number.

Examples:

• No estoy del todo seguro = said by a man
• No estoy del todo segura = said by a woman
• No estamos del todo seguros = said by a group of men or a mixed group
• No estamos del todo seguras = said by a group of women

The verb also changes:

• estoy = I am
• estás = you are
• está = he/she is
• estamos = we are

Because Spanish usually places no directly before the conjugated verb to make a sentence negative.

So:

• Estoy seguro = I’m sure
• No estoy seguro = I’m not sure

This is one of the most basic and regular negation patterns in Spanish.

Yes, more or less. La respuesta means the answer, referring to a specific answer already understood from context.

For example, if you are talking about a test question, a fact, or someone’s problem, la respuesta points to the particular answer being discussed.

If you wanted to say an answer, you would use:

• una respuesta

But in this sentence, la respuesta sounds the most natural because it usually refers to a specific one.

Yes, but it means something slightly different.

• No sé la respuesta = I don’t know the answer
• No estoy del todo seguro de la respuesta = I’m not completely sure of the answer

The first means you do not know it. The second suggests you may have an idea or an answer, but you are not fully confident it is correct.

So the second is less absolute.

Yes, it is the same word, but the meaning depends on context.

Seguro can mean:

• sure / certain
• safe / secure
• also insurance as a noun in other contexts

Examples:

• Estoy seguro = I’m sure
• Es un lugar seguro = It’s a safe place
• el seguro del coche = car insurance

In your sentence, seguro clearly means sure / certain.

Yes. You can replace it with other nouns, pronouns, infinitives, or even a clause.

Examples:

• No estoy del todo seguro de eso = I’m not completely sure about that
• No estoy del todo seguro de mi decisión = I’m not completely sure about my decision
• No estoy del todo seguro de haber entendido = I’m not completely sure I understood
• No estoy del todo seguro de que sea correcto = I’m not completely sure it’s correct

This is a very useful pattern:

• No estar del todo seguro de...

It sounds natural and polite. It is a fairly neutral, standard sentence.

In everyday speech in Spain, people might also say:

• No estoy muy seguro de la respuesta = I’m not very sure of the answer
• No estoy totalmente seguro de la respuesta = I’m not totally sure of the answer
• No tengo muy clara la respuesta = I’m not very clear about the answer

But your sentence is perfectly normal and idiomatic.

• seguro is stressed on the gu syllable: se-GU-ro
• respuesta is stressed on the pues syllable: res-PUES-ta

Neither word needs a written accent because they follow normal Spanish stress rules.

A rough pronunciation guide:

• No estoy del todo seguro de la respuesta
• no es-TOY del TO-do se-GU-ro de la res-PUES-ta

In Spain, respuesta is pronounced clearly with the s sounds, and estoy has the typical diphthong sound in -oy.