Linking verbs express no action, they express a state of being. They link the subject of the sentence to some information about the subject. The completion of the sentence comes from a complement: most likely a predicate noun which will rename the subject; or a predicate adjective which will describe the subject.
All forms for "to be" are linking verbs when they are the MAIN verb of the verb phrase:
am, is are, was, were, be, being, been
I am tired. "am" links the subject, I, to an adjective about me, that I am tired.
Sue has been the president of the club. "been" links the subject, Sue, to a predicate noun, president, that renames the subject. [Has is a helping verb which serves to clarify tense.]
There is another list of verbs that are sometimes linking verbs and sometimes action verbs. In these sentences you must consider whether the subject of the sentence is actually DOING the verb (action verb), or if the subject is merely being linked to a description. Here is a list of these verbs:
appear, look, feel, prove, grow, remain, taste, smell, turn, sound, stay, seem, become
I look tired. -- linking verb. The subject, I, is linked to a description of me, that I look tired.
I look for good deals when I shop. --action verb. The subject of sentence is actually doing the verb. I use my eyes to look around the store.
I tasted everything at the party. -- action verb. The subject of the sentence is actually putting food in his mouth and tasting it.
The food tasted really good. -- linking verb. The subject isn't doing anything; the verb is merely linking the subject to a description.
I could smell her perfume. -- action verb
Her perfume smelled awful. -- linking verb
I grow tomatoes in the garden. -- action verb
I grow weary of the student's complaints. -- linking verb
I felt the raindrops on my head. -- action verb
I felt sure of my win. -- linking verb
If you are referring to linking verbs, then I think I can help. Linking verbs are verbs that show no action. They are never verbs that can be done. Instead, these kinds of verbs make connections. They link the subject to another word in the sentence. The word the subject links to (the complement) always comes after the linking verb.
It was a sad day.
"Was" is the linking verb which links the subject (It) to the complement (day).
I always tell my students that if you can replace the verb with the equal sign (=), it's a linking verb. For instance:
It = a sad day.
The meaning of the sentence doesn't change, so you know that "was" is a linking verb.
Linking verbs are also called copulas. They link the subject to the complement. The complement could be a noun (I respect teachers.), an adjective (Anita looks beautiful.), or an adverb (The ladder rests against the wall.) The most common linking verbs are: appear, seem, look, be, turn, become, etc.