Sentence:  I begin by arguing in section 2 that there are in fact at least two Celtic <WSD>copulas</WSD>, a grammatical <WSD>copula</WSD> that simply spells out tense and agreement, and a substantive <WSD>copula</WSD> formed on a lexically listed verbal stem. 
Meaning of the word marked with <WSD> tags:  A word, usually a verb, used to link the subject of a sentence with a predicate (usually a subject complement or an adverbial), that unites or associates the subject with the predicate.