[Sublymonal]

I've recently purchased the Rosetta Stone program for learning Japanese. I did this after much deliberation [after all, it did cost 400 US$ and I'm operating on a student's income], and now find myself enjoying the experience.

However, I do have some issues. I'm still in the beginning stages, and I've come across the section that teaches the numbers 1-6. My question is: what is the significance of the different endings to the numbers? Or, in other words, when do I use the different endings?

There seemed to be a particularly large number of different endings for "san" [3], including: San, Sanko, Sandai, Sanmai, Sansatsu, Sanbiki, and Sanbon.

I know that with the Rosetta Stone program you're supposed to learn by example, but I just can't get past this. Thanks in advance for any help you may offer!