create trigger tg_yourname
on    dbo.A
FOR    INSERT
AS
BEGIN
    insert into B( 
        a1,b1,c1,d1    )
    select            
        a,b,c,d
    from    inserted x
END

------解决方案--------------------
随手敲的,难免手误:)
SQL code
CREATE TRIGGER insertAfterINSERT
ON tA
FOR UPDATE
AS
    IF UPDATE(a)
        INSERT tB(a1,b1,c1,d1) SELECT a,b,c,d FROM INSERTED

------解决方案--------------------
楼主的意思是说,你在更新a的时候,是有条件控制的吧,
比如说update A set a=b where id=2这样的更新语句,如果没有条件的话,会把所有的a全给修改.

create trigger tr
on A
for update
as
begin
insert into B (a1,b1,c1,d1) select a,b,c,d FROM A where id=2
end