Why we are not supposed to use about after discussed?
Because "discuss" doesn't take preposition. Probably you use a preposition in your language after "discuss" and when you want to translate that phrase / sentence into English, you expect a preposition to appear. No, it doesn't take preposition as:
We had discussed everything before the manager arrived.
But we say discuss something with somebody.
I discussed that matter with my close friend.
I didn't know the fact that we have to put examples in quotation marks.
Many thanks.
Remember that a lot of the learners have a basic level and it may be their first time using a forum like this.
If I were a new visitor and I read, for example ...
I don't know how to use these because it looks a lot like this and I also have a problem knowing when to use them
... I would have trouble knowing what it meant. Equally, if a learner posted that, the native speakers and teachers would have a problem knowing exactly what the question is. It's much easier for everyone if it reads:
I don't know how to use "these" because it sounds a lot like "this", and I also have a problem knowing when to use "them".