It's a little more complicated than that. American and British English can diverge because of differences in pronunciation. Say, "union", does not start with a semi-vowel for most Americans so it is not uncommon to see them pair it with "an.

The difference between "have had" (present perfect tense) and "had" (simple past) can be tricky to explain but as a rule, the present perfect indicates something that is still going on while the simple past indicates something that is definitely in the past. Compare:
I had a dog when I was a child (I no longer have it).
I have had this dog for the last five years (I still have it).
I lived there ten years ago. (past)
I have lived there for the past few years (I still do).
Sometimes the distinction is a bit harder to make. Compare:
They worked together.
They have worked together (they may not any longer do so but the consequences are still relevant today - so in a way, the effects are still going on).

"Have" refers to the present so it is a lot easier to distinguish from a past tense.
I have a dog. (now)
I had a dog. (back then)

True. Besides, it would have to be "thee" rather than thou. "Shall I compare thee to a Summer's day? Thou art etc."

Ah yes, Old and Early Middle English... Back when English, German and Dutch were almost identical. Too bad the French had too tamper with that nice arrangement, it would have saved me a few euros in terms of dictionaries

Ok, but it does not make it a rule to follow. Each language has some standards that should be followed. The fact that language is a living organism is another issue, but until most of the Americans consistently say 'an union/an university', I don't see the need to revise the phonological/grammatical rules. Then there's another question whether a language grammatical/phonological standards should be prescriptive or descriptive. Language evolves every day, but does it make it right to change the rules because eg. 60% of the population says 'He do..' I do not know.

I really like Mike Skinner's (The Streets) approach to the differences between BrE and AmE:

...or just the bits you got wrong

Definitely but then what are "rules"? "Mistakes" made by native speakers (other than young children) are rarely mistakes, they simply arise from rules that characterize a different geographical area or a different social stratum. They become mistakes only when made in contexts where they are not at home. It's all about pragmatics - and pragmatics is all about numbers, money, power and influence. So if the 60% are poor and powerless enough, then no.

Sounds familiar. There is a similar gap between Netherlands Dutch and Belgian Dutch. Two worlds living next door to each other. To make matters worse, we can argue forever about who invented the language because we used to be a single country.

It would be interesting to find out what exactly those rules are. For example, one could argue that the reason why people say 'he get' or 'innit?' (regardless of the personal pronoun at the beginning of the sentence) could be that they are eg. lazy. I wouldn't say it's true. It's more complex than that. It would be an overgeneralisation to say that most uneducated people are lazy or most people from Essex are lazy. There must be some other factors affecting the speech patterns of a particular area or a social stratum.

Let's not bicker and argue about who killed who. We can safely assume that it was the Proto-Indo-Europeans that 'invented' the language.

Hold your horses there, I did not make any references to laziness. What I really meant is that different communities have different grammatical rules. Why they should be different is indeed rather complex. One factor would be the centrifugal movement of linguistic development (linguistic communities have a center of innovation; the further one moves from that center, the older the language - for example, as the center of English was London until well into the twentieth century, American English is essentially seventeenth century British English). Another would be immigration, people trying to figure out the rules of their adopted language and coming up with something that is actually more logical. Why not say "he do" when English has already dropped the distinction between first and second person singular and between singular and plural? Why preserve exceptions to the rule? Then there are considerations of social distinction. At this time, I notice that more and more speakers of Dutch here in Belgium are shifting from a rolling r to a velar r (French style) just as more and more people in Holland are shifting from velar r to a retroflex r (British style). Why? Apparently because these pronunciations have recently been heard in popular tv series about the upper classes and people of distinction. I do not think these explanations are exhaustive but it is not as if we have not got any clue at all.

It was just an example for the sake of the argument.
Fair enough, especially that the general tendency over the last 1000 years has been towards simplification of the inflectional endings.

But it's not lazy to use && || instead of spelling AND OR out? Programming in C might seem incredibly lazy to some people, compared to, say, a more verbose language like Pascal or BASIC.

And while am at it, are you sure you're not just being a cryptic snob when you turn your nose up at anyone who doesn't pronounce SCSI the way you do? Just wondering...

That's completely different. It's like saying 'ad' instead of 'advertisement' or "I'd" instead of "I would", which is perfectly correct and within the rules of the language.

Again, we are not speaking of different languages. Any European language might SEEM incredibly lazy to Chinese. It doesn't prove anything as long as you follow the rules of the language you acquired.

Anyway, using the term 'lazy' wasn't a very fortunate choice for me. As I said, it was just an imaginary example of a possible way of reasoning, so please don't stick to it.
What's SCSI anyway?

Thanks jay73!

"Perfectly correct and within the rules". Unlike texting or leet speak I suppose.

So the key is to have the authority to make up your own rules. Then you're in control of what's right and wrong. Or in this case, correct and incorrect.

Thanks for straightening that out. I thought all these seemingly arbitrary rules had something to do with language but now I see it's really about obedience.