as to know where to start. I have in the middle of the lawn a large
green acer

what d'you mean by 'large'? could you give a (rough) height?

(Name not known) which i would like to be able to shape as
it has become overgrown. Does anyone know if you can cut them or will
it kill it?

you could formatively prune almost any tree as long as the operation is
carried out when the tree won't mind so much. and as long as the cut is
made by someone who knows what method won't cause later problems for the
tree.

having said that, some trees *always* mind. all acers i have encountered
have been deciduous, and can therefore stand some cutting when they are
dormant - in winter. although it might not be wise to lop the main trunk of
a 30m specimen to 10m, even in january ;-)