knitting question

[http://users.livejournal.com/_tiltedtiara_/]

i'm currently in brazil and its harder for me to find the same yarns as there are in the US

i want to use some of the yarns from here but its really hard for me to tell how to substitute yarns for different patterns properly.

when a pattern calls for a certain yarn you dont HAVE to use that yarn right? how do you know what other kinds of yarn will work? is there a general rule of thumb or a site or am i just not getting it??

thanks for any help!

-lauren