Hi Sasha, "Sick", "Old", "Unemployed" and "Poor" are indeed adjectives; however, they take up the role of nouns as well, depending on how the sentence is written. For example, when we write "The unemployed receive some aid.", we are referring to a collective group of people who do not have jobs. Similarly, "The poor" refers to a collective group of people who have another characteristic -- having little wealth. As we are referring to a specific group of people with a certain characteristic, the definite article "The" should precede these words. Reference URLs: http://www.dictionary.com/browse/poor?s=t (Look under definition #15.) http://www.dictionary.com/browse/sick?s=t (Look under definition #14.) The word "Teachers" itself is a noun and refers to people belonging to a specific profession, and the focus changes depending on whether "The" is used. (1) The teachers work hard. --> Your friend was referring to a particular group of teachers. Probably your friend and you both know the teachers whom you are referring to, and "The" indicates a "common ground" of understanding between your friend and you. (2) Teachers work hard. --> On the contrary, if you are referring to a common characteristic of many teachers, we do not use "The" and we pluralise the noun as well. Therefore, it would be a "mistake" to use "The" if you are referring to a common characteristic of something, but "The" is needed when you are focusing on a subset of a larger group of similar things. Examples: Lemons provide vitamin C. (No "The", a common characteristic of all lemons.) The lemon I ate was not sour at all. ("The" is used, focusing on one lemon you ate. Most likely, both the reader and the you know which lemon you ate.) Hope this helps you.

Sick, poor, old are adjectives. They act like nouns when you put "the" in front of them. You do it to identify a group of people with a certain characteristic, so "the" is a must (it's just an adjective without "the"). In the case with "the teachers", another rule applies - you already have a noun and you have to show whether you're talking about teachers in general or about some particular teachers.