رضا: Learning American English from movies? I'd take what you learn that way with a grain of salt. They can purposely use non-standrad English just for dramatic affect.

رضا: The English speak a different language than us Americans. I've heard this little joke about it.

Two countries seperated by a common language.

رضا: It used to be called "Proper English," but it's been awhile since I heard anyone say that. We just say, "English." The United States has no legal official language for most things, even though everyone says English is the official language. Someone wanting to become an American has to prove they know English, but if you're already an American you can speak any language you want to. And advertise in any language and exclude English from said advertising. Recently a city enacted an ordinance requiring English to be printed on a business's sign. It is being fought in court. I know Canada has such a requirement in Quebec, but that's a different country. I believe a business should be able to advertise in any language. If I can't understand their sign, then they're not wanting me for a customer. Seems simple enough to me. The only entity that I'd require to just use one language for official business is the government. Having a common language makes it easier for everyone here to know what to expect when dealing with the government and encourages people to know that language.

رضا: That site of Marfitalu's is in Canadian English. Perhaps they use it differently there? I know when I've travelled there some words are pronounced differently.

I'd say your book has it right, and it agrees with Marfitalu's and my use of the words when it comes to buying bread and bringing it home for the missus.

What's an infinitive? n. A verb form expressing action or condition without reference to person, tense, or number, as "to run". In English, its sign "to" is omitted after most auxiliaries, as in, "He should 'go' now," but is retained when the infinitive functions as a noun, as in "'To ride' horses was his favorite sport."

I'd be hard pressed to give you an example of an infinitve and feel positive that I was right from this definition! :)
English must be one hard language to learn. Somehow I missed all this and learned without it. I have the feeling some of you 'English as a Second Language' people probably have better English skills than some native speakers, though in reza's case it might be because he's an English teacher.

رضا: As I said, I'd use the words as Marfitalu has. If she is going to be upset, then "must" it is.

There's one thing about this whole deal, when it comes to which word to use. Most people around here could care less as to which is used. These meaning for these uses are somewhat subtle in their differences. I doubt if anyone would even notice in a conversation which one you had used unless you added special emphasis to it while you were talking. Other words can get people worked up. And then there's obscure ones like the word "whom". You got me who uses it, but there really is a proper way as to when to use it. I think the word "whom" will eventually disappear from American English.

One I was a kid, saying "ain't" was considered very bad. Not so much now. It's a right handy contraction.

رضا: I like how Marfitalu has it. If it ain't a problem but something that needs doing eventually, I'd go with "have to" or "should" or even "ought to". If it's urgent and can't wait or be denied, I'd be more inclined to say "must".

So we get back to the traffic light and external authority. We must obey the traffic signals, but some people don't think that way and would say, "We should obey the traffic signals". To me the word "must" is often times an absolute and I avoid speaking in absolute unless it really is an absolute. It should be obvious that we should obey traffic signals, we don't have to obey them. There's no must to it. A law of physics is different. It is an absolute in almost all cases. You jump off the ground, you must come back down. It has nothing to do with having to do it or should do it. It will happen and it must happen. Must implies little or no choice in the matter and the other ones leave room for different actions.

رضا: "something" is a word with lots of meanings too. If you wanted to use it to mean a type of fill-in-the-blank instead as some unspecified action for someone to do, you should have explained it in a different way.

Marfitalu: He says it well this Mister Micawber:

"Many authorities consider that must indicates an internal decision of the speaker, while have to indicates the presumption of an outward authority, but in practice these often overlap so extensively that either will serve."

He says they're often interchangable, but do have different uses.

In your first sentence I might use either one.
In the second I would normally say the I have to buy the bread, since it really doesn't have to happen usually. Again though, you could say must buy a loaf. Both make sense.

I'm trying hard to think up an example where one of them would be definitively wrong. Maybe someone else can. I speak my own version of American English and am not school taught in the fine points of a lot of grammar.

رضا: As far as I use the words, they're interchangable in your example. Have to and must do not mean the same thing though. Have is a tricky word and has lots of meanings. Must is fairly straight forward. You left out "got" by the way.

When do we got to do something?

This isn't normally an acceptable usage as far as grammartarians will say, but like the word "ain't" it is often said.

Is grammartarians a word? :)