=> It is
11) Samuel started keeping a diary in 1997
Samuel has
12)We started learning English five years ago.
We have
13) I began living in Ho Chi Minh City when I was 8.
I have
14) She has taught the children in this remote village for five months
She started
15) He has not written to me since April.
The last time
16) She hasn't had a swim for five years
She last
17) They last talked to each other two months ago
They haven't
18) The last time she saw her elder sister was in 1999
She hasn't
me for years