Look at the following examples:
The head-initialized antecedent (italicized) has a complementizer (bolded) that is the syntactic head of a full clause, the embedded clause (coloured amber).
The complementizer together with the embedded clause (coloured amber) performs the role of a complement, a subordinate clause that functions as the subject or the object of particular verbs. There can be a relationship, called a filler-gap dependency, between a wh- complementizer and a gap:
A Harvard University paper by Wilcox, Ethan, et al. (2018) stated that “filler–gap dependency refers to a relationship between a filler, which is a wh-complementizer such as ‘what’ or ‘who’, and a gap, which is an empty syntactic position licensed by the filler.”Wilcox, E., Levy, R., Morita, T., & Futrell, R. (2018, August 31). What do RNN language models learn about filler–gap dependencies? ACL Anthology. https://aclanthology.org/W18-5423/.
SUMMARY/OBSERVATIONS: The first two sentences have a that-clause that functions as an appositive or what is known as an expletive because the that-clause does not serve a grammatical function. In the third sentence, there is a filler-gap dependency, wherein the wh- interrogative, or complementizer, introduces an embedded clause. The word what fills a gap created by the verb, i.e., to be what. And in the last sentence, the entire clause functions as a subject, i.e., She sipped what looked like orange juice.