"Το πουκάμισο του είναι ανοιχτό."

Translation: His shirt is open.

[Dmatafonov]

why His shirt is light incorrect?

[troll1995]

If you wanted to talk about light color, you would say ανοιχτόχρωμο or ανοιχτό + the color p.e ανοιχτό κόκκινο= light red. Light with the meaning of "not heavy" is ελαφρύ. Ανοιχτό means light only when followed by a color.

[jasonalcant]

Disco Stu (or perhaps Discus Stu)?

[goofygoatgirl7]

it says 'light' on the translation. How can that not be correct?

[D_..]

Please read troll1995's comment above.

[Renardo_11]

Isn't του a clitic possessive? So could πουκάμισο get a second accent on the last syllable?

Sorry, I would just like to understand the rule.

[mizinamo]

Isn't του a clitic possessive?

Yes, it is.

So could πουκάμισο get a second accent on the last syllable?

It should.

There are many sentences from the beginning of the course where they do not have correct double accents because that form was not available to them while first writing the course; this seems to be one of them.

Sorry, I would just like to understand the rule.

Looks like you've understood it correctly :) This sentence is simply wrong.

[Renardo_11]

→ Mizinamo: Ευχαριστώ (και ρουμπίνι) / sinceran dankon (kaj rubenon)!