the job done first thing this year

I was stuck by teaching math today. I taught simultaneous equations ,though...before that my student seemed not understand how to put plus/minus sign when the numbers are transposed to the right/ left side of a mathematical equation. I couldn't explain when she asked me why the sign are changed like that. I got them very naturally and I never thought about it such deeply. And I also have to tel her know the meaning of each word, 代入(substitution)　項(term),　値(value) and so on. I really wonder what do the teachers teach their students teach in the school. Well, that's why I can get the second job like this even such economic slump in Japan,though...