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DON'T PRACTICE LANGUAGE WHILE YOU SHOP

The problem at the store started because I wanted to practice my Czech. However, the shop assistant wanted to practice her English.
I asked for coffee in Czech, so she poured a cup of hot coffee, placed it on the counter and tossed a package of ground coffee beside it. 
"A coffee and some coffee," she said.
I tried the Czech for potato and got a bag of potatoes plus a box of instant mashed potatoes.
"Serve the potatoes individually and offer each person a potato," she lectured. "If they are mashed, you must offer them some potatoes."
A request in Czech for tuna got me several cans of fish. "Some tuna," she announced. "Or I could say three cans of tuna."
How fortunate for me that the tuna is a very large fish. If I had been stupid enough to ask for a trout, she probably would have tossed a whole, live fish on the counter. Similar problems followed my requests in Czech for milk, rice, noodles and peas. 
"A liter of milk, a kilogram of rice, a package of noodles and two cans of peas," she said, displaying her ability to make uncountable nouns countable. "I don't just work as a shop assistant. I also give information about the correct use of English. Please notice that I said information, not informations, which would be very, very wrong!"
I needed chocolate as comfort, so I asked in Czech for chocolate. I got two chocolate bars and a gift box.
"Some chocolate and some chocolates," she said.
"There are two hairs on your blouse," I said to her in English, just to hurt her feelings.
"Your hair looks terrible today," she retorted.
I need more Czech lessons or another shop.

Questions

Choose the best explanations...
  1. Why does the sales assistant say information, not informations? 
  2. What is the difference between some chocolate and some chocolates?
  3. What is the difference between hair and hairs? 
  4. Why must you offer people some potatoes if you serve them mashed?
  5. Why does the sales assistant talk about liters and kilograms?
Are the following statements about the article correct?

1.   The narrator wants to eat an entire fish.

2.   Having terrible hair is worse than hairs on your blouse.

3.   Coffee sold as a drink in a cup can be "a coffee".

4.   The shop assistant speaks English well.

5.   It is never correct to say "a chocolate".