Another question :
How do I use the word " materials " and " material " ( singular ) . Many grammar books( written by American & British authors ) that I have referred to stated that " materials " could only be used for physical materials ,e.g. blood sample , paper ,etc . But the word " material" ( without "s" is only used for reading or academic materials.For example ,teaching material .
The explanations given by the books that I read were very confusing because I came across many academic writings that used " materials " instead of " material " for academic writings .
Perhaps, our experts overthere could englighten me on this word ( material). What is the ' safest' way of using material ?