1\An impacted wisdom tooth is one that fails to completely r

1
An impacted wisdom tooth is one that fails to completely rise through the gums -- the term is erupt.
the term is erupt.
2
Experts say people should have their mouths examined between the ages of sixteen and twenty for placement of their wisdom teeth.
for placement of their wisdom teeth.在句中充当状语对吗?
3
Scientists say the diet of prehistoric humans probably required more chewing teeth.Life was probably a little rougher on the teeth back then,too.So it was good to have extras.
Life was probably a little rougher on the teeth back then,too.
So it was good to have extras.to have extras 在句中充当什么成份?
4
After surgery,there can be swelling of the gums and face and some pain.
there can be 这是个什么句型呀,从没见过?