I like to draw a UML sequence diagram to display the sequence of actions and interactions of different parts of my system (I think they called actors).
However, I need to include if then else statements here. Do I do something wrong? Can I use conditional if statements in my UML sequence diagram? How?
As in many types of UML diagrams, you can use guards to denote a condition. A guard is a boolean expression that you can write over a transition.
Here's a picture with an example:
Notice the "[pastDueBalance = 0]" condition.
You can also use combined fragments for more complicated conditional logic. See this nice msdn article for further information.
There are four possibilities I know of (with increasing power):
My choice would be the least powerful one that is still sufficiently expressible for what you want to do. For instance, actions in 4. can have side-effects, which makes them much more fragile than OCL.
While sequence diagrams are perfect for your description, make sure to check Activity Diagrams.
When you want to show internal details (including detailed rules), Activity Diagrams is what you want to use. The plain activity diagrams focus on sequence of activities and rules and are poor in showing Actor responsibilities, nevertheless, you can use swim lanes to hi-light individual actor's responsibilities. Each lane would include activities an actor is responsible for.
These links discusses activity diagrams with swim lanes:
