Почему пишется grandpa а не grandfather

Grandpa - это разговорное слово, на русский может переводиться как "дед". 
Grandfather - это уже нормальное слово, которое переводится как "дедушка".

My favorite time of year - лето.В this year I have visited Armenia,a wonderful country with ancient history and hospitable people., I visited several places of interest of Armenia,well, let's say in Garni,Garni is old Armenian fortress and the settlement of 27 km to the East of Еревана.Побывала in Echmiadzin Echmiadzin city in Armenia, 20 km from Yerevan. Here is the most ancient monastery, the residence of the Catholicos of all Armenians, the spiritual center of Armenian Apostolic Church.(by the way, you can read about the Church of Echmiadzin very wonderful story!).I repeatedly been in Armenia,a very beautiful and elegant country chic history,this summer, I remember a lot,and especially the history of the Churches of Armenia!

1 Alice isn’t interested in looking for a new job.

2 Henry is excited about leaving for India.

3 You are capable of doing better work.

4 I have no excuse of being late.

5 I’m accustomed to having a big breakfast.

6 The rain prevented us from completing the work.

7 Fred is always complaining about having a headache.

8 Instead of studying, Margaret went to a ballgame with some of the friends.

9 Thank you for helping me to carry my suitcases.

10 Mrs. Grant insisted on knowing the whole truth.

11 I believe in being honest at all times.

12 You should take advantage of living here.

13 Fatima had a good reason for not going to class yesterday.