Student or Learner
You should think simply.
You should think simple.
Q. Simple isn't and cannot be a flat adverb and, therefore, it should be "think simply," right? Or is it still possible to use "think simple" because think can be used as a linking verb and thus, simple can be used as an adjective (v+ adj)?
Grammatically and technically you're right. However, in soundbites and taglines, you will often see such constructions.
Eat healthy. (Grammatically, it should be "Eat healthily" or "Eat healthy foods".)
Think big. (Grammatically incorrect but used a lot.
There is some logic behind it, I think. If someone was trying to come up with a concept or a product and they want it to be appealing to a lot of people, to be something that people all over the world want to use or buy, their target audience is "big". If someone asked them to describe the think they are thinking about, they might call it "big".
John: I want you to come up with an advertising campaign to sell our new shoes.
Tim: OK. What sort of campaign do you want me to do?
John: Well, we want to appeal to millions of people so ... don't think on a small scale, think on a really big scale.
Tim: Right. So don't think small, think big. Am I right?
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.