Think of modal verbs as a volume knob for instructions. The same basic message — "drink water" — can be whispered as gentle advice or shouted as an unbreakable rule, just by changing the modal in front of it. "You should drink water" is a friendly nudge; "You must drink water" is doctor's orders. The verb after it never changes — only the modal turns the volume up or down.
| Modal | Strength | Example |
|---|---|---|
| should | soft advice | You should drink more water. |
| ought to | moral advice | You ought to respect your elders. |
| had better | stronger warning | You had better wear a helmet. |
| must | strong obligation | You must wear your seatbelt. |
| have to / has to | external rule | Students have to wear uniforms. |
| must not / mustn't | prohibition | You must not smoke in the hospital. |
Some of the most important English in the world is one word long: Don't. To turn any command into a warning, bolt Do not or Don't onto the front of the bare verb. That's why this shape covers the world — every platform edge and park gate carries a "Don't…" sign, because it stops danger in the fewest possible words. The subject "you" is understood, so the sign skips straight to the warning.
When asking rather than ordering, soften it with Please don't: "Please don't bring food into the library." Same shape, gentler tone. And notice the three command patterns parents and staff switch between without thinking: a positive imperative ("Sit down"), a negative imperative ("Don't walk on other platforms"), and Let's + verb for a shared suggestion ("Let's go").
An adjective dresses up a noun (a quick runner); an adverb dresses up a verb — it tells you how the action happened (she runs quickly). When a safety poster says "cross the road carefully", that "-ly" word tells you not just what to do but how.
| Adjective | Adverb | Rule |
|---|---|---|
| quick | quickly | add -ly |
| happy | happily | y → i, then -ly |
| true | truly | drop the final -e |
| good | well | completely different word |
| fast / hard / late | same form | no -ly needed |
A "how" adverb usually sits after the verb or its object: "She speaks clearly." "He drove the car carefully."
Safety notice (40–50 words): leans on this unit's grammar directly — a strong modal for the unbendable rule, a quick run of commands for the steps, one sharp "Do not" for the danger, a human reason, and a signature.
Speech / poster (~100 words): opens with a surprising fact and a felt consequence, lines up the Do's and Don'ts in the same parallel shape (easy to remember on a poster), and closes with a call to action.