Sentence:  Tooth deformity may be the result of excessive thumb - or finger-sucking , tongue-thrusting , or lip-sucking - but it <WSD>'s</WSD> important to remember that there 's a difference between normal and excessive sucking habits . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)