Andy in Sig said:It's actually not easy once you start thinking about it because daily useage has such a strong effect.
Consider this: "That's me", is what most people would say, simply because it involves the verb to be. Grammatically speaking it should be "That is I". Similarly "It is he" as opposed to "It is him".
Now that I've written this, I've started getting confused myself because the "is" clearly relates to the "that".
I've just thought of another example. You will occasionally here "Those are they" whereas most people would say "that's them" and few of us would find that to be wrong.
A lot of what is held to be correct in English is based on rules which were simply invented in the seventeenth and eighteenth centuries by grammarians who took latin as a model because being classical it couldn't be wrong could it. So whereas you definitely can't split an infinitive in latin, there's no earthly reason why you can't in English. I think the worst culprit for all this nonsense was a bloke called Sheridan who offered courses in "correct" English. Correctness being of course, defined by him.
Indeed when children start speaking the english at 1+ years old they tend to construct sentences that are, logically speaking, grammatically correct, but as Andy says above, the grammatical rules of our language are based on a mongrel model of Latin and Saxon and there are many exceptions to logical rules. That's why I find it a bit sad when parents have a go at their kids for 'not speaking right' even though the kid is, in a way, correct.