“shell shocked” is an idiom, it originally comes from wartime when soldiers were psychologically shocked by the constant explosions from exploding artillery shells. You can’t combine “shell” separately with other words in the same way.
“I have a car.”
You own a car now.
“I had a car 4 years ago.”
I owned a car 4 years in the past, but I do not own one now.
“I have had a car for 4 years.”
You owned a car in the past, starting 4 years ago, and you still own a car now.
“When I was 20 years old I had had a car for 2 years.”
Same as “have had”, but the point of reference is some time in the past, not the present. So in this case you owned a car in the past, starting 2 years before you were 20, and at the time you were 20 you still owned it.
“I have been studying Hindi for six months.”
You began an action in the past and you are continuing to do it in the present, with the expectation that you will continue in the future.
“I had been in the room for 10 minutes when my brother walked in.”
The point of reference is in the past. You began an action before the point of reference and continued the action up to the point of reference.
“If I had been smarter, I would not have moved to Delhi.”
The is the past subjunctive tense. You are talking about a state of being in the past.
“I would have been happier if I had stayed in Assam.” “I would have been at work yesterday, but I was very sick so I stayed home.”
This is the past conditional tense. You want to say that there was some condition in the past that was not fulfilled.
Hope this helps.
