In this lesson, we’ll learn about the particle 도, which means too, also, indeed, even.

The particle 도 has 4 main features on its usage:

1.  It puts emphasis on the word before it, with reference to something earlier in the context.

2.  When you use the particle 도 with a noun, you do not use the particles that would mark that same noun as topic (은/는), subject (이/가), or direct object (을/를)

3.   However, when -도 comes with other particles, such as 에, 에게, 까지, etc., it takes a place after other particles.

4.  The particle 도 often occurs twice in a sentence, showing a kind of tandem agreement between noun phrases (both A and B do; neither A nor B does).

https://onlinekoreaned.wordpress.com/2022/07/07/the-particle-도