Learn Spanish (Latin America): Desde luego, tienes toda la razón.

While sí que is a great way to emphasize a verb (like saying "you do have..."), desde luego is a fixed phrase used to strongly agree or affirm a statement overall. It translates best to "certainly," "of course," or "without a doubt." It is another highly common tool for emphatic affirmation alongside the structures you are learning in this lesson.

Literally translated, desde luego looks like "from later." How does it mean "certainly"?

It is an old idiom. Historically, it meant doing something immediately. Over time, it evolved to mean that a fact is so obvious or true that it is accepted immediately without question—hence "of course" or "certainly." At this level, it is best to simply memorize desde luego as a fixed chunk rather than trying to translate its individual words.

Why do we say toda la razón instead of translating "completely right" using an adverb like completamente?

In Spanish, being "right" is expressed with the noun razón using the fixed phrase tener razón (literally "to have reason"). Because razón is a noun, you must modify it with an adjective like toda (all) rather than an adverb like completamente (completely). Saying "you have all the reason" is the most natural Spanish way to express that someone is completely right.

Could I say desde luego que in this sentence instead?