Sentence Correction

I was at the circus for only half an hour when the assailant struck;ever since the stabbing I am terribly afraid of clowns.

The correct solution is:
I had been at the circus for only half an hour when the assailant struck;since the stabbing I have been terribly afraid of clowns.

I fail to understand why the first part would be a past perfect....plz explain

Perhaps the following would seem more palatable to you:

I had been in US for only six months when the earthquake struck.

As opposed to:

I was in US for only six months when the earthquake struck.

Essentially there are two things that happened in the past: the fact that of my being in US for six months and the fact that earthquake struck. That being the case, almost always the "earlier" of the two events (my being in US for six months) is expressed as Past Perfect, while the "latter" event (earthquake struck) is expressed as Simple Past.

the concept of past perfect is that one action has ended and then the normal past event occurs.Therefore to my understanding when I was in US earthquaKe did not occur...

ruplun wrote:the concept of past perfect is that one action has ended and then the normal past event occurs.

That is not necessarily true. For example, both the following sentences are correct:

a) I had owned that watch for two years, before it needed a repair. - There is nothing to suggest that I do not own that watch now.

b) I had owned that watch for two years, before it got stolen. - Clearly I do not own that watch now.