Вкаких случаях и зачем ставиться в языке артикль "a"?

Неопределенный артикль "а" употребляется там, где где говорится о чем - то одном из многих предметов. допустим, ( i'm a child. я ребенок. один из многих) также этот артикль ставится там, где мы совсем ничего не знаем о предмете. (it's a dog) это собака. какая она - мы не знаем.

1

1. My sister is cleaning the kitchen now.

2. I often buy food on Fridays.

3. Nick didn't go to the Caucasus last year.

4. Look out! Who is watching you?

5. My grandfather usually works in the graden in the morning.

6. I did my homework last Monday.

7. Sometimes Maria doesn't cook lunch for herself.

8. Sorry! I am late.

9. The manager is busy. He is talking to a customer.

10. I was in the Crimea last summer.

2

1. Did I forget my umbrella?

2. Did my mother call me ten minutes ago?

3. Is the alpinist climbing the mountain?

4. Do I go to the gym every day?

5. Is my cat sleeping in his basket?

3

1. My elder sister doesn't play the violin every day.

2. Boys aren't playing football at the stadium.

3. I don't do shopping twice a week.

4. I didn't fly to Dublin last year.

5. My parents didn't go to China last winter.

1 Julie has sent/sent Mary several text messages

last night

2 Dave is going to recycle his old computer. He

has had/had it for over 10 years.

3 Have you ever used/Did you ever use Flickr to

share photographs

4 We have had/had a great time playing video

games at Paul's house last night.

5 Jane bought/has bought a new mobile phone

yesterday

6 I haven't received/didn't receive a reply from

the university yet.

7 Tara was/has been on the Internet since this

morning

8 How many emails have you sent/did you send

so far today?

9 Have you ever had/Did you ever have an

accident while text messaging?

10 Last Monday I have left/left the library at about 4:30 pm.