Here's a breakdown:
* Principle verbs express the main action or state of being. They are the verbs that you couldn't remove without drastically changing the sentence's meaning. Examples include: *run, jump, eat, think, be, seem.*
* Auxiliary verbs help principle verbs. They show tense, aspect, mood, or voice. Common auxiliary verbs are forms of *be, have, do, can, could, will, would, should, may, might, must.*
Examples illustrating the difference:
* "She is running." "Is" is an auxiliary verb (helping verb showing present continuous tense), and "running" is the principle verb (showing the action).
* "He has eaten the cake." "Has" is an auxiliary verb (perfect aspect), and "eaten" is the principle verb.
* "They will go to the park." "Will" is an auxiliary verb (future tense), and "go" is the principle verb.
* "I am happy." "Am" is an auxiliary verb (linking verb form of "to be"), and "happy" is a predicate adjective describing the subject, but "am" is the principle verb in the sentence.
In short, to identify the principle verb, look for the verb that carries the essential meaning of the sentence. Everything else helping it (tense, aspect, etc.) is an auxiliary verb.