From Cambridge Dictionary, Meaning of "lesson" [ C ]
a period of time in which a person is taught about a subject or how to do something
From Cambridge Dictionary, Meaning of "material" [ C or U ]
information used when writing something such as a book, or information produced in various forms to help people or to advertise products
EF is an online English learning platform which sells lessons. An EF lesson consists of some questions, answers and a vocabulary list containing dozens of words or phrases regarding a specific topic and usually lasts on hour.
I guess learning material in this context could refer to a vocabulary list in an EF lesson or the whole lesson in general.
A friend asked me about the platform yesterday, because I had been its paid classes. Here is part of my reply.
Most of the material in the EF lessons are not very practical. What kind of people would use personal checks in daily life?
I used the word "material" as an uncountable noun and "lesson" as countable. Did I use the correct forms?
Should I have used "material" in its plural form?
Most of the materials in the EF lessons are not very practical …
Best Answer
In this case both are correct. 'Material' can be both countable and uncountable.
If used as uncountable, your are treating the material as a mass. If countable, then you have treated it as divided up... perhaps into different categories: questions, answers, instructions, etc.