High school teacher in my case. I knew a bit of BASIC before his lessons, but nothing major. I couldn't really see how to use it to make a whole program.

Then his TurboPascal lessons opened up a whole world of possibilities. Not only did I take in the basics of any programming language, I also saw how these little blocks could lead to whole applications. Then I applied what I learned to PHP for some class websites, and am happy to say I'm still coding PHP to this day. I'll be forever grateful to that teacher.