#1: Correct., but I prefer "Lack of sleep can make a teenager tired". The article "A" at the beginning of the sentence is optional. Using the singular is also optional but I think it sounds better. Even though it looks like you are talking about just one teenager, you are not. When you say "a teenager" you are talking about all teenagers. #2: wrong. You cannot cause an adjective. You cannot cause "happy". You cannot cause "tired". You can only cause a noun or a noun clause like "to become tired". Here are some examples: His bad driving caused an accident. ("accident" is a noun) His laziness caused trouble. ("trouble" is a noun) The noise caused her to lose sleep. ("her" is an indirect object and "to lose sleep" is the direct object and a noun clause)

I think both of these are correct. do not affect my understanding

Hi Molly, You can say: "Lack of sleep can cause teenagers to be tired" or "lack of sleep can make teenagers tired" General rule : to cause something to be + adjective / to make something + adjective