A conditional statement is an if-then statement. For every conditional statement you can write three related statements, the converse, the inverse, and the contrapositive. The video shows how these are related.
If you can write a true statement using "if and only if" , then you've got a biconditional. Learn more about biconditionals by watching the video.
When a conditional statement and its converse are both true, you can write them as one statement called a biconditional statement. Learn more about this special kind of statement by following along with this tutorial.
This tutorial looks at using an indirect proof to show that an equilateral triangle cannot have a right angle.