The most important verb no doubt is To BE. It occupies a central position among the verbs. It involves identity, expresses something about a person, thing, a state (gives: age, condition, date, position, price, size, time, weight) or refers to temporary and permanent states. It is taken for granted and left out (Arabic and Russian) in abbreviated styles. This is perhaps the reason why it is so short and its conjugation is so odd and most irregular. After all its meaning denotes existence which is basic in human life. It expresses questions and answers about existence: there is/are; is there? Compare the popular line in Shakespeare's Hamlet "To be or not to be". However it can have a variety of meanings: two plus two is (equals) four.
Regular verbs, also called weak verbs, stick to a pattern. Irregular verbs i.e. strong verbs live up to their name and follow different patterns and might be a pitfall even for writers. Verb "to be" is indeed an exception. It has neither the regularity of regular verbs nor is subject to one of the patterns of conjugation as with irregular verbs. In addition, it has a very high frequency. It can be a main verb: I am happy or can help to form a compound tense as an auxiliary as with continuous forms: She is writing. Like other helping verbs it helps to make questions or question tags: are you interested? Furthermore, although "to be" is mostly used in a stative sense it can be used dynamically: He is being nasty to her. It is a linking verb (copular verb) as well because it takes an adjective and not an adverb: She is tidy. Verb "to be" helps to form the passive voice: The window was broken.
"Be" combined with other words makes phrases and idiomatic expressions. Most idioms with "be" contain nouns and adjectives. With "used to" it can even change the meaning and the infinitive to gerund compare: I used to smoke - with - I am used to smoking. It can join "able" to make up for the deficiency of "can": I will be able to do it. "Be to" conveys a plan, gives orders, it can be an obligation or refers to destiny: Students are to be here by 8 O'clock. "Be" used as a prefix (although a bit old) in certain words has the meaning to make or treat sb/sth i.e. make intransitive verbs (without object) transitive (with object: belittle, befriend, bemoan your lot - or used in past participles: bespectacled, bejewelled. Perhaps a variety of words which start with "be" now lexicalized, were coined with the help of "to be". "Be" is often used with "it" as an impersonal object: it was very noisy there. Just consult any good dictionary to find out how versatile and crucial this verb is.
In translations verb "to be" can be rendered by "to have" depending upon the language translated into:
It's my birthday today
Ich habe heute Geburtstag
You are right
Du hast Recht
What's the date today?
Den wievielten haben wir heute?
What's wrong with you?
Was hast Du?
"To BE" as a Shortcut
In spite of the importance of "verb to be" it can be substituted by other verbs: I am Tom - my parents named me Tom. This made followers of E Prime (English Prime: based on General Semantics) plead for abolishing or eliminating it altogether and calling for semantic hygiene. They claim that the ideas of identity and predication (attributes) associated with "to be" are misleading, vague, objective, absolute, abbreviated and therefore controversial because like present simple verbs they imply no time, space but absolute truth:
"John is sad" can imply John is always sad, John was always sad. John will be sad and he can do nothing about it. They suggest replacing or translating such "be sentences" into E Prime: John appears to be sad here today. Other examples:
I am an engineer.
I have a degree in engineering.
That is a sexist film.
That seemed like a sexist film to me.
This vagueness they claim is detrimental to our way of thinking and perception basing their ideas on linguistic determinism (Sapir-Whorf-Korzybski Hypothesis). However, this type of thinking seems to ignore pragmatics. If I say "I am sad" people usually know what's meant by it depending upon its context i.e. the situation in which it was said. Ambiguity can only arise when a statement is removed from its social, spatial and temporal background. In addition, not all "be sentences" are ambiguous: This is female/male - is crystal clear. "To be" 's power and beauty lie its brevity and in the fact it is a shortcut which avoids long and boring sentences suggested by E Prime followers. Precise language is after all boring, direct and mathematical. Just imagine no more: playing on words, poetry, saying something but meaning something else. Ambiguity in language is its beauty and power.