Why we say "I am doing the shopping"? why not "I am doing shopping" like "I am going shopping"?
The two statements have different implications:
I am out shopping right now.
While this is perfectly correct as a catch-all for any shopping activity; it is most commonly used to describe non-specific shopping where you are browsing stores. For example, when going shopping for clothing or other luxury goods - it would be common to spend the entire day "shopping" (going in and out of shops, seeing what they have and making purchases).
If there's one specific type of item you have in mind, it would be relatively common to state it as:
I am out phone/clothes/wine shopping.
I am out shopping for phones/clothes/wine.
However, the phrase:
I am out doing the shopping right now.
Tends to imply more specifically that you are completing a regular grocery shopping. It wouldn't be incorrect to use this phrase when out shopping for clothing (or other goods), but it gives the impression that this is part of your regular schedule and you are simple completing it like a chore.
Well, like most "why" questions about language, the real answer is "because that's what we say".
But we can delve into the patterns of English a bit.
"Shopping" in "go shopping" is not a noun like "laundry" or "chores": it is a participle of the verb "to shop". It happens that "go" can be followed directly by a participle, denoting an activity that will happen while going, or after arriving somewhere. Examples: "I went swimming yesterday." "Are you going clubbing?". "Come" can also take this pattern, but I can't think of any other verbs which do.
In particular, "do" does not take this pattern, so "doing shopping" does not work, and we use a different construction, with the noun "shopping".
The vs. a, an, some
The article the indicates a sometimes subtle signal that the object in question is an instance of something that you and your conversation partner are already familiar with as a possible topic of discussion:
When you use the there is an expectation that the listener knows which one. For example, imagine you answer the door and a policeman is there. You say to your partner, who is sitting in the next room:
The policeman is at the door!
The fact that you said the policeman implies that you expect your partner already knows to which policeman you are referring. Perhaps you called the police earlier and were expecting a visit. In this case, the policeman refers to a particular instance of the set of all possible policemen. On the other hand, suppose you were not expecting a policeman to visit your house. In that instance, you would say to your partner:
A policeman is at the door!
By saying a policeman you are introducing the fact that there now exists an instance of a policeman here to which you can refer. From now on, you may say the policeman to make further statements about that same policeman:
The policeman wants to talk to you!
What do you do if you don't want to refer to a particular object? For countable nouns, you normally use an indefinite article 'a' or 'an', as appropriate:
For noncountable nouns you normally would use the word 'some' or add a countable quality along with an indefinite article 'a' or 'an'. For noncountable nouns you may also use the null article (no article at all):
a piece of furniture
a lot of furniture
two hundred milliliters of water
Once of you have introduced such an object, you may now say the furniture, and your listener will understand it to refer to the previously introduced instance and quantity of that noun. For example the water would refer to the same 200 milliliters of water that I indicated earlier. If I say the sugar after only mentioning sugar earlier, you may have to guess based on context. If the two speakers have not quite reached agreement about the actual instance or amount, you might be able to hear this based on the articles they choose. For example, imagine a customer at a coffee shop asking for sugar:
Customer: "Do you have some sugar?"
Clerk: "Here is some sugar."
Customer: "Thank you for the sugar."
In this fictitious conversation, the customer used some sugar to refer to some unstated quantity of sugar in his head, and it seems the clerk returned to the man with some amount of sugar that she supposed might qualify as some sugar, but she probably realized, at least subconciously, that the amount of sugar that she chose may not necessarily be the exact same amount that the man had in mind. Still, she was pretty sure that the amount she chose would still suffice, so she didn't bother to ask him exactly how much he wanted. For this reason she returned with her chosen amount of sugar and said to him some sugar to refer to the quantity that she was handing him. Now that both speakers are aware of this sugar, it has now become an established topic, and the man refers to this sugar as the sugar when he thanks her. In another situation, suppose you are buying a part for your bicycle at a shop:
Shopkeep: "This part costs twelve dollars."
You reach into your pocket and count twelve dollars.
You: "Here is the money."
Why did you say the money in this situation? Because you and the shopkeep already established the amount of money. If you had said some money in this situation, it would give a strange signal to your conversation partner, implying that you and he hadn't yet established how much you should be giving him. Did you not hear him say that he wanted twelve dollars?
The shopping, the laundry, and so on
Why we say "I am doing the shopping"?
Based on the above explanation, the reason you say the shopping, the laundry and so on when talking about chores is that you and your conversation partner have previously established by some means that those things are something that you might refer to. You and your partner already expect that shopping is an instance of something that you might do, probably because you do it on a recurring basis. So when you say
I am doing the shopping.
You are referring to the particular instance of the action of shopping that you both have already established. Perhaps you have a shared To-Do list in your house where you write down the chores to be done:
The shopping refers to the shopping on this list. Even if you don't actually have such a list on paper, you probably have something resembling this list in your head, or you can imagine that such a list might exist based on common conventions done by many people in today's society.
The ABC Presentation
The above explanation will work the same no matter the topics involved, even if they are not a regular task, and even if they involve things other than common activities like shopping, laundry, and so on. For example, suppose you work in an office and you see the following items on the team agenda:
- XYZ Presentation: 2:00pm
- Cost comparison 3:00pm
- ABC Presentation 4:00pm
- Closeout 5:00pm
You and your colleagues share the agenda, which means you should all know about those items as possible topics of discussion. For this reason it would be quite natural for a colleague to approach you and ask:
Who's doing the ABC presentation?
Then you would say:
I'm doing the ABC presentation.
Or you would use a shorter form such as I'm doing it or I am. However if you had replied,
** I'm doing an ABC presentation.
Your colleague would feel puzzled. He just asked you were if you are doing the ABC presentation, but you said you were doing an ABC presentation. Do you mean a different one than he means?
Now, suppose a remote colleague calls you. Suppose your remote colleague has access to the shared agenda. Then it would be quite natural to refer to that presentation in the same way:
Remote colleague: Hi, Joe.
You: Hi, Bob.
Bob: Can we do a teleconference at four?
You: No. I'm doing the ABC Presentation.
If Bob didn't know about the presentation, he might respond with 'oh, is there an ABC presentation today?' or if he knew about the presentation but didn't know you were giving it he might say 'oh, are you doing the ABC presentation?'
Now, suppose a customer calls you. You have no reason to believe that she knows about your agenda. She probably also has no idea what ABC and XYZ presentations are, because they are specific to your internal organization.
Customer: Hi, Joe.
You: Hi, Alice.
Alice: Can we do lunch at two?
You: No. I have a meeting. Can we do lunch at one?