Modal: Should

Should is the voice of gentle guidance. It tells you what is advisable, expected, or morally right — without the weight of an absolute command. Where must says "you have no choice," should says "this is the sensible path, but the decision is yours." You should get a good map of London is friendly advice; You must get a good map of London is an urgent warning.

The primary use of should is giving advice and recommendations. It is the most natural modal for this purpose — softer than must, more confident than could. In British English, should can even be used for first-person polite advice: I should apologize to her if I were you, meaning roughly the same as you should.

Should also expresses logical expectation. When you say She should be here by now, you are not stating a fact; you are stating what you expect based on what you know — her usual punctuality, the bus schedule, the time she left. The negative form works the same way: The package shouldn't take more than three days expresses your expectation, not a rule. Compare this with must for certainty: She must be here by now carries stronger conviction, while she should allows room for doubt.

In conditional sentences, should adds a formal, cautious tone. If you should need anything, please call is more deferential than the plain if you need. You can even drop the if and invert: Should you need help, do not hesitate to ask. This inverted form is common in formal written English.

Beyond advice and expectation, should can express surprise — It's strange that he should say that — and it appears in purpose clauses with so that, in order that, for fear that, and lest: I wrote it down so that I shouldn't forget. In all these uses, should stays true to its nature: it nudges, it suggests, it expects — but it never forces.