I used to get confused when speaking I don't know whether put should or must
Quote: "Must you decide? You should you know. Should is a suggestion, must is an order."
Not quite that easy. The reference below:
takes you to "Fowler's: The King's English" of which I have the 1951 edition - and here's what he says:
"Thou shalt not steal" is the type of shall in the pure system. We do not ordinarily issue commands to ourselves; consequently I shall is hardly required; but we often ask for orders, and therefore shall I? is required. The form of the shall present in the pure system is accordingly:
Shall I? You shall. He shall. Shall we? They shall.
As to the past tense, orders cannot be given, but may be asked about, so that, for instance, What should I do? (i.e., What was I to do?) can be done all through interrogatively.
In the conditionals, both statement and question can be done all through. I can give orders to my imaginary, though not to my actual self. I cannot say (as a command) I shall do it; but I can say, as a conditional command, I should do it.
I shall and we shall are accordingly the superfluous forms of the present shall in the pure system.
Again, with will, I will meaning it is my will, it is obvious that we can generally state this only of ourselves; we do not know the inside of other people's minds, but we can ask about it. The present runs, then,
I will. Will you? Will he? We will. Will they?
The past tense can here be done all through, both positively and interrogatively. For though we cannot tell other people's present will, we can often infer their past will from their actions. So (I was asked, but) I would not, and Why would I do it? all through. And similarly in the conditionals, I would not (if I could), &c.
The spare forms supplied by the present will, then, are you will, he will, they will; and these, with I shall, we shall, are ready, when the simple future is required, to construct it out of.
Sir Bruce Fraser. in the 1984 edition of "Plain Words" talks of the Celtic (Irish, Scots, Welsh). He Says:
But the idiom of the Celts is different. They have never recognized "I shall go". For them "I will go" is the plain future. The story is a very old one of the drowning Scot who was misunderstood by the English onlookers and left to his fate because he cried "I will drown and nobody shall save me".
With hindsight it occurs to me that this rather gnomic joke, which doubtless has pedantic grammarians falling about with laughter, requires some further explanation:
The Celt, in this case a Scot, does not differentiate between "shall" and "will". As far as he is concerned, his meaning is "I am going to drown unless somebody saves me". As far as the grammatically correct English are concerned, however, what he is shouting is "It is my will to drown" followed by the imperative "nobody shall save me" - so they leave him to it.
So should & would are conditionals but there is no conditional for must
"You shall go to the ball Cinderella!"
"Oh! Sorry, you should have gone to the ball but we couldn't find the mice or the pumpkin."
"You must go to the ball Cinderella!"
"Oh! Err - sorry!"
Hope this helps