"To be" is the most irregular verb in modern English, as well as the most common. In fact, it is a collective verb in all the Germanic languages. It takes eight different forms in Modern English:

BE = infinitive; subjunctive; imperative.
AM = Present, 1st person singular.
ARE = Present, 2nd person singular and all plural.
IS = Present, 3rd person singular.
WAS = Past, 1st and 3rd persons singular.
WERE = Past, 2nd person singular, all plural; subjunctive.
BEING = Progressive & present participle; gerund.
BEEN = Perfect participle.

These represent the merger of three once-distinct verbs:

S-root = eom, eart, is, sindon
B-root = beo, bist, biš, beoš
W-root = wesan, węs, węron.

Beon was the only Anglo-Saxon verb with a special future form. It was used specifically for future states of being, or "coming to be," or statements of eternal truth. It comes from an Indo-European root meaning "to dwell."

The S-root form traces back to another Indo-European root, the same one that gave the Greek esti- and Latin est (and German ist). The Old English form was esan.

For plural and subjunctive, Old English could also use sindon (sind, sie), which died out in the 12th century and was replaced by "are" (eart), though it continues in Modern German as the 3rd person plural of "to be."

The past-tense forms (was, were) come from Old English wesan, and they are the only Modern english survival of the Verner's Law shift of "s" into "r" between infinitive and 2nd preterite. This transformation once affected a great many English verbs. Wesan also means dwell/remain, but didn't have the sense of permanence of beon. The imperative, wes, is buried in "wassail" = wes hal, or "be healthy." Beon was generally reserved for permanent states, esan and wesan for temporary ones. The use of the w-root as a preterite goes back to Proto-Germanic. Gothic has was, us, im. "The incorporation of the b-root into this group seems to be a West Germanic innovation."

The picture is further clouded because Old English had strong regional variations among the various kingdoms of the island, and different combinations were used in Northumbria than in the West Saxon kingdom. For instance, West Saxon used beoš as the plural, but the modern plural, "are," is descended from the Anglian earon.

Retention of the b-root form was a mark of conservative dialect until the 1950s in rural West Country England -- "I be," "you be," "they be." It is a badge of non-standard grammar in contemporary America.

Be, have, do, and go are the most common verbs in most languages, and the most irregular. They often double as helping verbs (auxiliaries). Their roots in existence, possession, action, and motion are, some linguists say, the essence of all verbs.

Be and go are the only Modern English verbs whose past tenses are different words. "Went" is from "wend," which survives, barely, as a present-tense verb only in the stilted construction "wend one's way," meaning "go." Oddly, the Old English "go" also had a past tense from an entirely different verb than its present, but not "wend."

Most Old English adjectives took the ending -ra and -ost for comparative and superlative. Thus leof (dear) became leofra (dearer) and leofost (dearest). But the most common comparatives had highly irregular endings:
YFEL (bad) - wiersa, wierst.
GOD (good) - betra/selra, betst/selest.

Betra and betst derived from bot, an Old English word meaning "remedy, reparation." It survives in one phrase, "to boot," meaning "in addition." You can see the connection between this word and "good" in our use of "better" to mean simply a return to normal ("It's all better.") The other O.E. word that had a parallel function, selig, has an even odder survival in Modern English, though in German it still means "blessed, happy." From this notion of "blessedness" still present in German, it passed in English to mean "innocent," and thence "naive," and finally to "silly," which is how it is used now (terminal "g" became "y" fairly early in English).

Bad is something of a mystery word: it doesn't appear in English until the end of the 13th century and it has no apparent relatives in other languages. It just possibly comes from two derogatory terms for homosexuals, będdel and będling, in Old English.

Evil, the older word, has grown worse in meaning as "bad" took its place as the opposite of "good," and "extreme moral wickedness" came to be its principal meaning.

Worse and worst go back to prehistoric Germanic wersizon, a comparative form of wers- which also gave English "war."