I saw the play Women Beware Women at the National this weekend, so it gave me the idea to write about the strange verb 'beware'.
Beware is a so-called defective verb, which means it does not have the usual tenses, aspects and moods. You cannot put beware in the past tense, or say 'I am bewaring'. Beware is usually used in the imperative (Beware of the dog) and can also be used in the infinitive (You must beware of the dog, I reminded him to beware of the dog) and that's about it.
The reason for this goes back to Old English (ie before the Norman Conquest) when there was an adjective wær, which became ware in Middle English (after the Norman Conquest). It meant 'cautious' or 'on one's guard' so the expression used would be 'be ware' (two words). By 1300 it was often written as one word, and by 1600 endings were put on it by some erudite writers (bewares, bewared, bewaring). These eventually dropped out of fashion.
Thomas Middleton wrote Women Beware Women in the 1620s or thereabouts. The 'Beware' in the title is usually assumed to be an imperative, but it was in use as a finite verb too when Middleton was writing. Having now seen the play, I'm wondering if the title could be a play on words that could be taken two ways (imperative, present tense).
Another example of a defective verb is the archaic quoth, which means 'said' and only appears in the past tense. It once had a present form -- queath -- but this disappeared in the 15th century. Ought and other modal verbs (can, must, may etc) are usually considered defective verbs.