I'm learning devanagari now, and I'm confused: I think that what's shown to be the vowel mark for the long "u" in this course is actually for the short one, and vice versa.
For instance, look here: http://en.wikipedia.org/wiki/Devanagari
तुलसी & तू
the first one is short tu and the other one is long vowel just like tool
see the difference above.