have minimal semantic content but fulfill important grammatical functions, e.g. express contrasts involving tense (past, present, future), aspect (completed, ongoing), and modality (possiblity, probability, obligation)
carry information about tense and aspect (e.g. do, be, and have)
denote notions relating to possibility, probability, permission, obligation, etc.
reflect type of elements with which word can occur
co-occurence with determiners
individuals and objects (entities)
co-occurence with markers of tense (past, non-past, future) and aspect (completed, ongoing, repetitive, etc.)