Desde luego, estoy de acuerdo.

Desde luego adds emphasis, something like of course, certainly, or indeed. It makes the agreement sound stronger and more confident than just estoy de acuerdo on its own.

In Spain, desde luego is very common and natural in conversation.

No. Even though desde often means from in other contexts, desde luego is a fixed expression. In this sentence, you should learn it as a whole phrase meaning of course / certainly.

So here it is not being understood word by word. It functions as an idiomatic expression.

Because Spanish uses estar de acuerdo for to agree.

Even though English uses to be in I am in agreement, Spanish specifically uses estar, not ser, in this expression:

• Estoy de acuerdo = I agree
• ¿Estás de acuerdo? = Do you agree?

This is just the standard structure you need to memorize.

De acuerdo means in agreement or simply okay / agreed, depending on context.

For example:

• Estoy de acuerdo = I agree
• De acuerdo = Okay / all right / agreed

So in your sentence, estoy de acuerdo literally means something like I am in agreement.

Yes, often you can.

• De acuerdo. = Okay / Agreed.
• Estoy de acuerdo. = I agree.

De acuerdo is shorter and very common in conversation.
Estoy de acuerdo sounds a little fuller and more explicit.

With desde luego, both are possible, but Desde luego, estoy de acuerdo sounds especially clear and emphatic.

A simple Spain Spanish pronunciation guide would be:

DEHZ-deh loo-EH-go, es-TOY deh ah-KWEHR-doh

A few useful points:

• desde: the d can sound softer than in English
• ll in luego in most of Spain sounds like an English y
• estoy has the stress on -toy
• acuerdo has the stress on -cuer-

If you want a more natural rhythm, the sentence usually flows quite smoothly: Desde luego | estoy de acuerdo

Because desde luego is being used as a sentence adverb or introductory expression, similar to of course in English.

So the comma helps separate it from the main statement:

• Desde luego, estoy de acuerdo.
• Of course, I agree.

In casual writing, people do not always punctuate perfectly, but the comma is standard and helpful.

It is fairly neutral. It works in both informal and many formal situations.

• With friends: completely natural
• In a meeting: also natural
• In writing: acceptable, depending on tone

It is not slang, and it is not overly formal. It is a standard, everyday way to express clear agreement.

Yes. It sounds natural in Spain.

Both parts are common:

• Desde luego
• Estoy de acuerdo

A speaker in Spain might also say similar things such as:

• Claro, estoy de acuerdo.
• Sí, estoy de acuerdo.
• Totalmente de acuerdo.

But Desde luego, estoy de acuerdo is perfectly normal and idiomatic.

Sometimes, yes. Because it adds certainty and emphasis, it can sound stronger than a simple yes or I agree.

Compare:

• Sí, estoy de acuerdo. = straightforward agreement
• Desde luego, estoy de acuerdo. = more emphatic, like Absolutely, I agree

So if you want a softer or more neutral tone, you might choose sí or just estoy de acuerdo.

Yes: Desde luego, yo estoy de acuerdo.

But usually yo is not necessary, because estoy already tells you the subject is I.

Adding yo may:

• give extra emphasis
• create contrast, as in I agree, but others may not

In most normal contexts, Desde luego, estoy de acuerdo sounds more natural than including yo.

Yes, several. Depending on tone, you might hear:

• Claro, estoy de acuerdo. = Of course, I agree.
• Por supuesto, estoy de acuerdo. = Of course / certainly, I agree.
• Sí, estoy de acuerdo. = Yes, I agree.
• Totalmente de acuerdo. = Totally agree / completely agree.

Compared with these, desde luego often sounds confident and idiomatic, especially in Spain.