1.Verify whether it looks like a pen or not? Here u need to test it's basic look and feel properties.2. Verify whether the pen cap functioning well? Likely test whether all of it's parts working properly or not?3. Verify whether it's writing properly or not?4. Verify it's working condtions on a piece of paper? All these are functionality test cases.5. Verify it's working condtions on a piece of wood,floor,..etc? To explore negative cases.6. Test whether it breaks if u apply external force to break it? negative caseYou can list out a few cases by analyzing like this.
SanityTesting is an initial technique appllied on initial build.This is to make sure that whether the build is working properly or not, to conduct further detailed testing on on the product.If the build functions well..we'll apply system testing on it ,if not we'll reject the build.Genarally people will do this by preparing a Sanity Check List.Smoke testing is also similar to Sanity.But if u detected that the build is not working properly and rejected the build then u would have mentioned the reasons(atleast a guess) for it's improper functionality. i.e. here u r guessing(ofcourse rough idea)something about the improper functionality of the build.