Dear Lenka:
You are not alone in being puzzled by some aspects of English Grammar.
Lynne (Cunliffe) makes some valuable points.
When people have a written or spoken conversation, (more especially when it is spoken), there are certain conventions. Native speakers generally follow these conventions, from habit.
For example, they take turns at speaking, first Person A, then Person B, A, B, and so on. It saves time, it helps conversation, and it is a polite way of exchanging information.
They don�t CONSCIOUSLY think about any convention � they just DO IT.
This applies to ALL LANGUAGES.
Between a Speaker and a Listener, the context �says� many unspoken things. For example, if I enter your Bank, YOU automatically know that I�m going to talk about MONEY, and NOT to ask if you sell FISH. This �unsaid information� is automatically �understood� by both persons.
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Now to your letter.
You are correct regarding sausage/onion/cucumber sandwiches. In normal situations, we NEVER put a whole sausage, onion, or cucumber in a SANDWICH.
This �unsaid information� is automatically �understood� by both persons.
(A �hot dog� is a special kind of food, which is a whole sausage in a bun, very often served at Barbecues.)
In any case, if you are in a Party, Restaurant or Sandwich Shop, you can SEE what type of sandwich you are getting. This information is not given, it is �unsaid�. But, it is �understood� by the speaking individuals.
GIVING INFORMATION:
Customers frequently say, "Thank you!" when they receive information, service, food, etc.
�We have several sandwiches! We have tuna/lettuce/egg mayonnaise/pork/bacon, lettuce and tomato/salmon/mushrooms/tomato/gherkins, etc. with white bread/brown bread/ wholemeal bread/etc.�
�Salt, pepper, ketchup/serviettes/plates/knives, forks and spoons are on the table, in the corner�.
(Different people like or dislike ketchup, or garlic, for example, so the restaurant doesn�t NORMALLY put it in the sandwich. IF IT IS in the sandwich, THEY SAY it is in the sandwich!)
�Oh, look, my friend! There�s tomatoes, eggs, pineapple, garlic and sliced beetroot in this sandwich!�
QUESTIONS, beginning with: �Excuse me!� and ending with �Please.�
�Have you/do you have � a chicken?/a salad?/a sausage?�. ���.�Yes, we have!�/No, we haven�t!�/ OR �Yes, we do!�/No, we don�t!�
Singular Sandwich, or Plural Sandwiches:
"Is this/are these � cheese/egg/banana?� ���. �Yes, it is!�/�No, it isn�t!� OR �Yes, they are!� OR �No, they aren�t!�
When there are very few sandwiches visible, or the shop will close soon:
�Is there any coleslaw/beef/bacon and lettuce available?� �Yes, there is!� OR �No. there isn�t!� OR �We have only one/two/three left, Sir/Madam�.
When there is an ingredient which you like/don�t like.
�Is there garlic/gherkin in this sandwich?� ���. �Yes, there is!� OR �No. there isn�t!�
�Is the gherkin chopped/sliced/whole?� �It�s chopped/sliced/whole/I don�t know. Let�s take a look � yes, it�s chopped/sliced/whole!�
�Miss, there are Peas in my Chicken Sandwich!� ���. �Sorry, I�ll bring you a fresh one!�
IN A RESTAURANT:
"Would you like ketchup/sauces/salad cream/dips/mayonnaise/etc. Sir/Madam?"
"Yes, please!/No, thank you/ ketchup, please!"
I repeat! Please, take note that much of the vocabulary is �unsaid�, but it is �understood� by everyone.
These include words and phrases, such as, �A sandwich in our Restaurant�; �all of our sandwiches�; �the sandwich which you can see in front of you�; �to eat with your sandwich�; �do you want?�; �some�; �any�; �less?�; �more?�; etc.
Most customers and staff are in a hurry and they want the transaction completed without wasting time talking.
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�Waiter, do you have Frog�s Legs?� ���. �No, Sir! I always walk this way!�
Lenka, there are lots of other possible answers but I hope that these help you.
All the Best!
Les Douglas