It can be confusing for you to know the difference between inferno and infernal in the beginning. When you say infernal, this means that you are referring to a place that looks like it is bordering on hellish or is already hellish. This can also refer to a large fire that might be happening somewhere. When you say inferno, this normally refers to a large fire in a certain place like various houses that are burning all at the same time.
At times, people use the term inferno in order to refer to hell itself. Some may even use this term when they would like to refer to something that seems to be almost diabolical.