甚麼是link verb?

In linguistics, a copula is a word used to link the subject of a sentence with a predicate (a subject complement or an adverbial). Although it might not itself express an action or condition, it serves to equate (or associate) the subject with the predicate. The word 'copula' originates from the Latin noun for a "link or tie" that connects two different things (for a short history of the copula see the appendix to Moro 1997 and references cited there).
A copula is sometimes (though not always) a verb or a verb-like part of speech. In English primary education grammar courses, a copula is often called a linking verb.
The term is generally used to refer to the main copular verb in the language: in the case of English, this is "to be". It can also be used to refer to all such verbs in the language: in that case, English copulas include "to be", "to become", "to get", "to feel", and "to seem". Other verbs have secondary uses as copulative verbs, as fall in "The zebra fell victim to the lion."
[edit] Use
We can identify several sub-uses of the copula:

Identity: "I only want to be myself." "When the area behind the dam fills, it will be a lake." "The Morning Star is the Evening Star." "Boys will be boys."
Class membership. To belong to a set or class: "She could be married." "Dogs are canines." "Moscow is a large city." Depending on one's point of view, all other uses can be considered derivatives of this use, including the following non-copular uses in English, as they all express a subset relationship.
Predication (property and relation attribution): "It hurts to be blue." "Will that house be big enough?" "The hen is next to the cockerel." "I am confused." Such attributes may also relate to temporary conditions as well as inherent qualities: "I will be tired after running." "Will you be going to the play tomorrow?" but please note that a linking verb has nothing to do with these so called "Be"- verbs.(see below)

[edit] Non-copular uses

As an auxiliary verb:

To form the passive voice: "I was told that you wanted to see me"
To add continuous aspect to tenses: "It is raining"
Meaning "to exist": "I want only to be, and that is enough." "There's no sense in making a scientific inquiry about what species the Loch Ness Monster is, without first establishing that the Loch Ness Monster indeed is." "To be or not to be, that is the question."
Note that the auxiliary verb function derives from the copular function; and, depending on one's point of view, one can still interpret the verb as a copula and the following verbal form as being adjectival. Abelard in his Dialectica made an argument against the idea that the copula can express existence based on a reductio ad absurdum (Kneale - Kneale 1962 and Moro 1997).

A linking verbs connects the subject of a sentence with a word in the predicate that identifies or describes the subject. TO BE in all its forms is the most common linking verb.
ex:::::
~George Washington WAS a general. (The linking verb WAS links general to the subject, George Washington.)
~The computer WILL BE useful. (The linking verb WILL BE links useful to the subject, computer.)
common linking verbs::::
appear, become, feel, grow, look, remain, seem, smell, sound, stay, taste, turn