(By Hedvig Skirgård): "Complementizers are markers (free or bound) which are able to turn a clause into the subject or object of a sentence. In some of the literature these are also known as ‘conjunctions’. The complementizer in the sentence I knew that Ashley was a genius is that and it is preposed to the complement clause (Ashley was a genius). The complementizer does not have to be phonologically free, the position in relation to the complement clause is what is relevant. This feature concerns whether the complementizer precedes the complement or not. ❡
This feature specifically targets sentences with verbs of thinking and knowing, where the complement clause is an object (I know that he is truthful or I think that this is a bad idea). This is done so that we get more comparable data across the sample."