An argument with all true premises and a false conclusion might be valid - is this true or false? Examples would be very helpful.

What you find on the following sites may be of help to you.

Validity - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Validity)

http://pegasus.cc.ucf.edu/~jmundale/BasicLogicalConcepts.html

Validity and soundness (http://philosophy.tamu.edu/~gary/intro/validargs.html)

Allen/Hand, Section 1.1 (http://aristotle.tamu.edu/~rasmith/Courses/Logic/Exercises/1.1.html)

Deduction Worksheet (http://facweb.bcc.ctc.edu/wpayne/dedwrk.htm)