Fill in a or an into the gaps
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Зад.1  watch-часы (наручные), один экземпляр..  watches - это несколько часов. Специфично слово - deer (олень, олени). Множественное число  этого слова совпадает с единственным, нет окончания (e)s. Еще слово teeth - мн. число образуется изменением корня (tooth-teeth, зуб-зубы)

Объяснение:

Правила произношения и написания окончаний существительных во множественном числе зависят от исходного слова. Общее правило - конечные звонкие согласные не оглушаются, то есть произносятся как z-iz. Вот тут коротко и ясно: https://engblog.ru/plural-of-nouns

Dear Robert,
I'm very glad to hear something new from you. You ask me about my school. Well, my school is large and light. There are two floors in it. There is a gym, a library, a canteen and a few classrooms on the ground floor. There are many classrooms and a teachers’ room оn the first and on the second floors. Our classrooms are very large and light. There are twelve desks, a teacher’s table, a blackboard and many flowers in our classrooms.
My favourite subjects are English and Chemistry.
My school life is wonderful. There are lots of concerts, performances and sport competitions in our school. My classmates and I often take part in different school events. I have a lot of friends in my class.
Our teachers are kind and intelligent and my classmates are smart and friendly.
Tell me a few words about you and your school and friends. Have you joined any sport club? Do you like your teachers?
That's all for me for today. I have to go to the swimming pool.
Best regards to your family.
Write soon.

надеюсь