VERBS, AUXILIARY. Should, "should have," Shakespearian use of"Should have" with the second and third persons. The use of "should have" with the second and third persons is to be noted. It there refers to the past, and the should simply gives a conditional force to "have." It is incongruous to use should in connection with the past, and hence we now say "If an angel had come" in this sense. When we use "should have," it refers to a question about the past which is to be answered in the future. "If he should have forgotten the key, how should we get out," i.e "if, when he comes, it should turn out that he had forgotten." Compare, on the other hand, the Shakespearian usage.
In M. Ado, ii. 3. 81, the "should have" is inserted, not in the conditional clause, but in a dependent relative clause. "If it had been a dog that should have howled thus, they would have killed him."
“Gods, if you
Should have ta'en vengeance on my faults, I never
Had lived to put on this.