Paul est tout à fait honnête.

Because the subject is Paul, which is third person singular.
The verb être changes depending on the subject:

• je suis = I am
• tu es = you are
• il/elle/on est = he/she/one is

Since Paul is like il, you say Paul est.

French sentence order here is very similar to English:

• Paul = subject
• est = verb
• tout à fait honnête = description of Paul

So the structure is:

Subject + form of être + adjective

That is why Paul est honnête means Paul is honest.

Tout à fait is an adverbial expression that strengthens what follows. In this sentence, it intensifies honnête.

So Paul est honnête = Paul is honest
and Paul est tout à fait honnête = Paul is completely / quite / absolutely honest, depending on context.

It is a fixed expression very often used for emphasis.

No. Even though the words literally suggest something like entirely / quite / totally, you usually should learn tout à fait as a set phrase.

Depending on context, it can mean things like:

• quite
• completely
• absolutely
• exactly
• yes, absolutely (in conversation)

So it is better to understand its function than to translate each word separately every time.

Because in tout à fait, tout is part of a fixed adverbial expression, not an adjective modifying a noun.

Here it does not agree with Paul or honnête.
So you keep:

• tout à fait honnête
• tout à fait sincère
• tout à fait prête
• tout à fait intelligents

The expression stays the same.

In French, adjectives used with être usually come after the verb, just like in English after to be:

• Paul est honnête
• Marie est contente
• Ils sont fatigués

This is called a predicate adjective: it describes the subject through the verb être.

There actually could be agreement in other contexts, but here Paul is masculine singular, and honnête already has the same written form in both masculine and feminine singular.

Examples:

• Paul est honnête
• Marie est honnête

These look the same in the singular.

But in the plural, you can see agreement more clearly:

• Paul et Marc sont honnêtes
• Marie et Anne sont honnêtes

So honnête does agree, but its singular masculine and feminine forms are identical.

Because the h in honnête is a silent h. More specifically, it behaves like an h muet.

That means French treats the word as if it begins with a vowel sound. So you can get normal vowel behavior with it.

For example, in connected speech, tout à fait honnête flows smoothly into honnête without a pronounced h sound.

Yes, usually. Since honnête begins with a silent h, it behaves like a vowel-initial word. So est honnête is commonly pronounced with liaison, roughly like eh ton-nête.

This is normal French pronunciation after est before a vowel sound or silent h.

Yes. Paul est honnête is a perfectly complete and natural sentence.

Adding tout à fait simply makes the statement stronger or more emphatic. Compare:

• Paul est honnête = Paul is honest
• Paul est tout à fait honnête = Paul is quite/completely/absolutely honest

So tout à fait is optional, but it changes the tone.

It is generally neutral and very common. You can use it in everyday speech and in writing.

It is not slang, and it is not especially formal. It is a normal expression that learners will hear often.

Yes. Très honnête is also correct and common.

The difference is mainly in nuance:

• très honnête = very honest
• tout à fait honnête = quite/completely/absolutely honest

In practice, both intensify honnête, but tout à fait can sound a bit more like entirely or absolutely, depending on context.

You could say C’est tout à fait honnête, but that would mean something slightly different depending on context.

• Paul est tout à fait honnête clearly says that Paul is honest.
• C’est tout à fait honnête usually means That is quite honest / perfectly fair / entirely honest, referring to a situation, statement, or thing.

So Paul est... is used because the subject is specifically Paul.