"The chair is for you."

Translation: Der Stuhl ist für dich.

[runtimean]

How can one distinguish between "dich" and "euch"?

[stepintime]

In this case, you can't.

One could argue that you normally offer one chair to one person (e.g. at a table), but this could also be about giving a chair to a couple for their household, so "für euch" works as well.

[Sociokai]

Why isn't it Der stuhl ist fur du

[mizinamo]

For the same reason that "The chair is for he" or "The chair is for I" would be wrong -- you need an object form, not a subject form. Thus dich and not du.

[woody620809]

Sessel also means chair?

[mizinamo]

Sessel also means chair?

Not in Germany. And this course teaches -- and expects -- German from Germany.