• Multiple auxiliaries can occur in a single sentence, while a matrix sentence may have at most one main verb.
• Auxiliary verbs cannot occur as the sole verb in the sentence, but must be followed by a main verb.
• All auxiliaries precede the main verb in verbal sequences.
• Auxiliaries do not subcategorize for any arguments.
• Auxiliaries impose requirements on the morphological form of the verbs that immediately follow them.
• Only auxiliary verbs invert in questions (with the sole exception in American English of main verb be).
• An auxiliary verb must precede sentential negation (e.g. John not goes).
• Auxiliaries can form contractions with subjects and negation (e.g. he'll, won't).