Marfitalu: But your choices are in contradiction with what that link says. According to the link, in the first one you must (?) use have to since it's an as the link puts it, outward authority, an din the second one, you must (?) use must since afterall it's an option.

رضا: I wonder how many native English speakers really know this difference and use it.

Fencer: My assumption is that many don't know the difference, but use the options properly.

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.

Walter Montego: Yes. The link works well. The fact that non-native speakers of a languag eknow the grammar of that language more than its native speakers do, is quite interesting. Though the language intuition is always something else.

Walter Montego: You said: 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.

So now that I looked back at my post, I started to wonder if I had put my question properly. By the word something in my examples, I simply meant that you as the ones who want to answer my question are free to put the proper actions in the place of 'something' that I used. But seems you as a native speaker didn't get what I meant. So my question must have been wrong.

Quite interesting that Farsi doesn't always correspond with English!

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.

Walter Montego: So this would work?

When do we say : I must buy a loaf of bread on my way back home.

And when : I have to buy a loaf of bread on my way back home.

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.