Suppose I teach English (at school or I'm a private teacher, or I'm writing a book, or I'm creating an app - the point is that people pay me). In my teaching materials I give examples of correct sentences written in English. In the world there are correct English sentences which are already said or written: in books, in songs, in vlogs, in the comments on StackExchange.
Can I use such sentences (in the written form) in my teaching materials without asking permission to or paying anyone? If so, must I put the source of the sentence? Let's assume I'd just take one sentence from one context (1). I'd like to spread my teaching materials internationally. Are there any catches?
(1) I mean that, for example, I wouldn't write "We are the champions, my friend. We'll keep on fighting till the end", I would write at most one of these sentences.