
Lauren F. answered 03/26/19
Proven Results! Spanish & ESL Tutor
Think of it this way...
The word "the" is a definite article. It is referring to a specific one.
The word "a" could be any one of those objects. Which can be a little confusing in your second example.
The man is lying on the floor.
The important idea is to think about the intent of the speaker or writer. If you say "the" man, it is implied that the listener knows which man. It is important that this message is about him, not just any man. The article "the" in "lying on the floor," means it is a specific floor and the listener knows which one.
If this were changed to, "A man is lying on a floor," it means that somewhere in the world, some man is lying on some floor. No specifics are known or are important to what I am saying.
He is standing on the chair.
Same as above. It is a particular chair and the listener understands which one.
The woman is writing a letter
The first word, "the" refers to a particular woman. We want the other person to know we don't just mean any woman. It is specifically the one we are looking at or where previously talking about.
Then, the word "a" is used in front of " letter". This means that as far as our conversation goes, it is not necessary for us to know specifically what letter she is writing. The fact that what she is doing is writing a letter is what is important,
She is reading a book.
Due to the use of the indefinite article, which book is not important to our conversation, and we may not even know which book. The important message is tell what she is doing. The important message of the speaker or writer is what action the person is doing.
The man is running. He is riding a bicycle.
In the above, we are referring to a specific man. Details about which bike he is riding are irrelevant in our conversation. If we say "the bike," it means we are talking about a particular bike. For example, maybe we were talking out a really old bike you lent him. That would be "the bike."
The woman is driving the car. The man is driving the car.
As you catch on to the idea, you understand that we are talking about two particular people. It doesn't mean we have to know everything about them, but these particular people are relevant to our conversation. The fact that they are driving "the" car means that the speaker refers to a specific car, such as the one in the picture.
I hope this makes sense! Please send any questions and I will try to answer!