Why does "I was happy to do my homework" work, but "I was tired to do my homework" doesn't?

Question

I'm teaching ESL, and I came across a question from one of my students that I don't know how to answer. Using the form "{subject} {verb} {adjective} {infinitive phrase}" we've been going over sentences such as "I was happy to help you." (More specifically, "I helped you. I was happy. -> I was happy to help you.") One of my students then suggested "I was tired to do my homework." Now, as a native English speaker, I know that this is wrong. I'm even college educated and actually trained in ESL (which included grammar classes)... and yet, I have no idea WHY this is wrong. It SEEMS to fit the form our textbook was teaching ("I did my homework. I was tired." -> "I was tired to do my homework.") and yet I know it's wrong. What's the difference? Why doesn't this work?

Answer 1

When you say that you are happy to do it, it means that you do not mind doing the homework. However, when you use tired, it conveys a feeling of dislike or unwillingness to do the homework. You would need to say "I was tired of doing my homework" or "I was tired after doing my homework". This shows that you became tired as an effect of doing the homework. Since they aren't necessarily happy because of the homework, I believe that this is why you can't use tired in this context.