I'm having a hard time understanding why most people consider the infinitive to be and all of its verb base forms helping verbs. I've consulted multiple English grammar sites and forums, and most of them classify to be a helping verb in certain situations. This starkly contrasts my beliefs, and I want to if someone can distinguish the differences, if any, for me.
Linking verbs such as be, seem, remain, appear, feel are NOT helping verbs. Why? Because helping verbs are doing exactly that: helping; they are HELPING other VERBS, and linking verbs themselves are not action verbs, but verbs expressing a change in state or appearance. Therefore, any form of be and all other forms of linking verbs mustn't be considered helping verbs because they equate one part of the sentence with another and cannot be considered a main verb. Linking verbs expand upon the meaning of a sentence, and no action is being performed.
Consider this sentence: "They have been developing new methods to facilitate in-vitro fertilization."
The subject of the sentence is they, the present perfect tense is indicated by the use of the helping verb have, and been is acting only as a linking verb; "developing new methods . . ." is merely the participial phrase, modifying the subject they. Even though a helping verb follows the linking verb, I believe the linking verb must not be considered a "main verb" itself. The past participle been is not functioning as the main verb of the sentence, nor is the present participle developing; been is linking two parts of the sentence, and developing begins the participial phrase that modifies the subject they.
Helping verbs make more apparent their job when they do not precede a linking verb, but a past participle. Consider this sentence: "I have broken the window."
The subject of the sentence is I, the present perfect tense is indicated by the use of the helping verb have, and broken is a past participle functioning as the MAIN VERB to complete the verb string have broken. In uses like this where there are no copular verbs, the past participle can very well be regarded as functioning as a main verb. There are no copular verbs, so nothing is being described or closely defined, nor is anything acting as a complement; broken really is the only "verb" of the sentence, and the job of the helping verb have acknowledges this. This contrasts with uses involving linking verbs. I've read sentences such as "He was eating the toasted sandwich," and people have said that the verb was is acting as a helping verb, but this is wrong. Was is a LINKING VERB, nothing else: it is linking the description, "eating the toasted sandwich," to the subject of the sentence, he. The people who've said that it is a helping verb only say that because they believe that was eating itself is a verb phrase, and that it is not describing him, but acting as the "verb" of the sentence. These people are considering the action being expressed more closely than the linking verb's describing the subject of the sentence. You DO use a form of to be and a present participle to form a past progressive ACTION, yes, but these words' grammatical functions are not actions because there is a linking verb involved, and copulars equate parts of sentences with each other. You are forming the idea of something progressing in the past, something HAPPENING, but since a linking verb is used in the formation of that very idea, it is not expressing action, but linking something with something else. In this case, was is describing something that the subject, he, was doing—"eating the sandwich"— and is conjugated for the past progressive, hence was and not is, will be, or is going to. Basically, the meaning of the sentence is action, but the formation of the sentence is description.
Present participles cannot act as verbs themselves and are strictly adjectives, excluding their being used as gerunds, in which case they are nouns. They always follow linking verbs or precede the word they modify. Take the present participle burning; "the burning building" and "the building is burning" both contain the present participle functioning as an adjective. Whether they come before the part of a sentence they modify or follow a copular verb, present participles are adjectives, unless used as a gerund. You cannot use a present participle as a "main verb" as you would a past participle: "I have eating this sandwich" makes no sense, but "I have eaten this sandwich" does; have is the helping verb, and eaten is functioning as the main verb to complete the verb string "have eaten." You can say "I am eating this sandwich" or "I have/had/will have been eating this sandwich," but the fact still remains that all of those aforementioned constructions involve linking verbs, and no direct action is being expressed even though the meanings of those sentences CONVEY ACTION.
I just want to be enlightened on this as I had always regarded copulars as verbs that link parts of sentences with each other, but having read differing views and inputs about the status of linking verbs in regard to their function, I don't know whether they can, in certain situations, be considered auxiliary or helping verbs. Until recently, I had never read about copulars acting as auxiliaries or helpers, so this is one big area of confusion for me. All that I have described is how I have come to think of linking verbs' grammatical functions, and this way really works easily for me, but I would appreciate if someone could help diffuse my confusion because this is something that has been wracking my mind for quite a while.