Beware means 'be careful — there is danger here'. What makes it worth a whole entry is that it is grammatically strange: it is a defective verb, meaning it has no other forms. There is no 'bewares', no 'bewared', no 'is bewaring'. You can only use it as a command — 'Beware!', 'Beware of the dog', 'Beware the ides of March' — or after another verb that needs the plain form: 'You should beware of cheap imitations', 'Travellers must beware of pickpockets'. Anything else is wrong. You may see it with 'of' or without. With 'of' is the everyday pattern and takes a noun: beware of the dog, beware of strangers, beware of anyone who promises quick money. Without 'of' is more literary and dramatic, and is how the famous lines go: 'Beware the ides of March', 'Beware the Jabberwock, my son'. You can also say 'beware that...' or 'beware how...' in formal writing. In tone it is stronger and more old-fashioned than 'be careful'. A parent says 'be careful' to a child; a sign on a gate says 'BEWARE'. It suits warnings about a real hazard — a dog, a cliff, a con artist — and is often used half-jokingly today for smaller things.