...Children become very selfish and they start to enjoy their own lives, I mean - they live just for themselves and don't think about their parents. What is going to happen next? And what is the basis of such (a) behaviour?
Should I put an article here?
NOT A TEACHER
The use of the phrases with "such" follows the normal rules of using indefinite articles. That is, since behavior is an uncountable noun, you can't use the indefinite article. If you had a countable noun after such, you would have to use an indefinite article. For example, "He's such a nuisance", "such a fast car", but "such advice", "such sheep" (advice and sheep are uncountable nouns).