You can use an asterisk as a "wild card" in your search entry: clemen* will find clement, clemens, clementine, etc., but that will generate a huge list of web sites most of which have nothing to do with Mark Twain (Samuel Langhorn Clemens).
But you know other things about Mark Twain that can help you search. You could, of course, just search for twain, but you would quickly find out that the Internet is flooded with information about a kind of sub-routine that links computer programs, which is also inconveniently named Twain. If you search for mark twain you will get a lot of web pages that contain the word mark (think of the gospels) plus all that computer subroutine stuff.
What you have to do is group the information that you know so you get what you want and avoid what is not relevant. Grouping is done with parentheses: ( ). Type in (twain AND samuel) and you are likely to get what you want. Type in (twain AND samuel) OR (mark AND clemen*) and be certain. Note that the connectors, AND and OR, must always be in capital letters. You can similarly ungroup information by using a NOT connector or a minus sign (-). Remember that the minus sign must be right in front of the word or words you want to eliminate -- no space follows the sign.
If you want to become expert at using these kinds of "Boolean" searches, it requires some study. A fairly simple explanation is at a web site called "American Literature and Online Search" (http://www.accd.edu/pac/english/amlit.html).
There are more detailed explanations at two web sites called "Beyond Surfing" (http://magi.com/%7Emmelick/it96jan.htm) and "Sink or Swim" (http://www.ouc.bc.ca/libr/connect96/search.htm.)