The article deals with complex sentences of a dissected structure, expressing investigative relations, in linguistics and in the practice of teaching RCTs. The paper analyzes the educational and methodological complex, which allowed us to state that in the practice of teaching RCTs, the most common is the conjunction-adverbs therefore, which is presented already at the initial stage in comparison with the conjunction because in order to form the skill of transforming one syntactic structure into another. In the course of the analysis, it was found that the only investigative union so that (according to classical linguistic science) at the initial and secondary stages of training, the RCT is not used. Also, at the level of general knowledge, there are no syntactic models with unions of differentiated values due to what, due to what, due to what, due to what.