What are Auxiliary Verbs?
Auxiliary verbs can be considered as add-ons in a clause which modify the meaning of it. They can add grammatical or functional meaning to the clause of a sentence that they appear in. Various parts of a sentence are modified by or associated with the use of an auxiliary verb. An auxiliary verb in a sentence can have its impact on the tense, modality, aspect, emphasis, voice, etc. of the sentence. Auxiliary verbs are usually written before the verb to impact the verb and tell the tense of the sentence. The main verb in a sentence represents the semantic of that clause in which it is being used. For example, if a student says that he has completed his homework, the verb is completed, and the auxiliary verb has. Here completed represents whatever the student is trying to imply, whereas has is telling about the tense in which the sentence is being said.
Use should to express the ideal (best) action which happens in the past, present, or future. The negative of should is should not or the contraction shouldn't. Use shall to express a future action. Shall is different than will in that you use it to express an order or prophecy. The negative of shall is shall not or the contraction shan't.