This is a very common problem type seen at the beginning of a course in organic chemistry, and as such they have a sneaky way making it onto exams. They are of the kind in which a question is asked such as, "Draw as many of the structural isomers of the formula C_XH_Y as you can."

Here I describe a systematic technique for drawing the isomers for saturated hydrocarbons (the alkanes), which have the formula C_NH_N+2. First I'll describe the technique, handled in five steps, and then I'll apply it to a problem to show you how to put it to practical use.

THE FIVE STEPS FOR DRAWING ISOMERS:

1. Draw the main chain.
   
   
2. Draw the main chain minus 1 carbon, and add a methyl group to as many positions as possible. Never add the methyl groups to the end of the chain, and watch not to repeat structures (it's okay if you accidentally repeat structures, for they will be caught and discarded when you do step 5).
   
   
3. Draw the main chain minus 2 carbons, and add two one-carbon groups (two methyls) or one 2-carbon group (an ethyl) to as many positions possible, trying not to repeat structures.
   
   
4. Continue subtracting and adding groups in this fashion until you run out of carbons or doing so only results in repeated structures.
   
   
5. Give the IUPAC name to all the compounds you drew. If you accidentally drew the same one twice, they will have identical names, and you can cross one of them off.
   

 See an example problem