One morning, a patient arrives at a doctor's office complaining of severe back pain.
The doctor examines him and asks, "Okay, what happened to your back?"
The patient says, "You know I work for a local nightclub, right?"
He continues, "This morning I came home early to my apartment and I heard a noise in my room. When I went in, I knew that someone had been with my wife, then I saw that the French-window to the balcony was open.
I hurried out onto the balcony but found no one. Looking down from the balcony, I saw a man running out of the building and putting his clothes on in a hurry.
"And?" The doctor says.
"I grabbed the fridge and threw it at him—that's how I sprained my back."
A second patient comes in like he's been in a car accident. The doctor says, "Good God! The previous guy looked bad, but you look very bad. What happened to you?"
The man says, "You know I've been unemployed for some time now. Today was the first day at my new job. I forgot to set my alarm clock and I was late. I was running out of the building, getting dressed at the same time, and you won't believe it but I got hit by a fridge".
A third patient arrives, looking even worse than the other two patients. The doctor is shocked, but asks, "What the hell happened to you?"
"Well", The third man says, "I was hiding in a fridge..." "I saw a man running out of the building and putting his clothes on in a hurry". The "a man running" is the past continuous tense, which we use to indicate a long action in the past, an action that began before a specific moment, and an action that continued after the specific moment. For example, we could say, "She was walking (past continuous) toward the bus-stop when you called her (past simple)".
She started walking before you called her. And she will continue to walk after you call her. I call the past continuous tense the alibi tense, because it is the one everyone will use when the police ask, "Where were you at the time of the murder?" ["I was playing football with my friends"].