The questions you are asking about Struts tags can all be answered by the Struts documentation.
There is a section for each of the various tag libraries in this link. For this question, look for the section entitled "Logic tags".
In answer to your specific question: Strus has an "if" (<logic:equals>) but no "else". That's why Vani and I both recommend using the JSTL tags over the Struts tags in this case. [ September 11, 2006: Message edited by: Merrill Higginson ]
With the logic tags you would have to use have to use two pairs of tags. You could use either a html:equal and html:notEqual pair or use a value="true" and value="false" pair. I agree that the JSTL tags are cleaner.