11 ** Complete the second sentence so that it means the same as the first.
1 I never liked pancakes, but now I do.
1 like pancakes, but now I do.
2 The last time I went to Serbia was 2009.
1 to Serbia since 2009.
3 You aren't allowed to attend without a
ticket.
You attend without a ticket.
4 I was watching the parade yesterday
when I saw Mark.
Yesterday, I saw Mark
watching the parade.
5 Are you fond of chocolate?
Do
chocolate?​

The nature and the person are closely connected. And if the nature can exist without person, then the person without it can't. Therefore it is so important to live in harmony with the nature.
But, unfortunately, people try to subordinate to themselves the nature, to become her owner, but so it is impossible. The nature needs to be protected, it is our house. Necessary for life the nature gives all to us. If was considered earlier, the natural riches which aren't settled that they in large quantities, then now it becomes clear what is in limited quantity much. The person can't use gifts of the nature constantly as they can disappear soon. A task of the person to preserve the nature and to increase her riches. It is necessary to approach the matter as we are obliged to leave behind to the descendants the rich, beautiful, and pure planet responsibly.

2 и 6 будут Past indefinite 
Остальные ,соответственно,- Present Perfect
I have built a house.
I have built a house when he came out of school
I have never built houses
I haven't built a house
Have you ever built a tower?
We built a house today
I have not yet learned words
We haven't made tutorials
They have never seen a crocodile
They haven't seen yesterday the crocodile

Так. На заметку.
Время Present Perfect обозначает действие, которое завершилось к настоящему моменту или завершено в период настоящего времени

Время Past Simple(indefinite) используется для обозначения действия, которое произошло в определенное время в и время совершения которого уже истекло