Modus Tollens, which is a form of propositional logic basically means that If A happens then B happens so if B doesn’t happen, then A probably didn’t happen.
An easy example of this would be if the first thing a person does when he gets home is put his keys in a bowl, if you see that his keys aren’t in the bowl, chances are he isn’t home.
The important thing to understand is while propositional logic is generally sound, it’s certainly not completely conclusive. Using the above example, if the persons keys aren’t in the bowl, it usually will mean he isn’t home BUT there can be exceptions. He might have lent his keys to someone else. Or, he might have had an emergency and forgot to put the keys in the bowl.
Modus tollens is a good way to make conclusions as long as you understand there are exceptions.
Have a great day!