What is "queismo"

Queísmo is a phenomenon in Spanish grammar, the omission of a preposition, usually de, which, in Standard Spanish, would precede the conjunction (or complementizer) que. For example, "No me di cuenta que habías venido" ("I didn't realize you had come"): compare standard "No me di cuenta de que habías venido". Queísmo is frowned upon by prescriptive grammar.

Although the omitted preposition is typically de, other prepositions occasionally are also subject to omission by queísmo: "Insisto que te vayas" ("I insist that you go"); compare standard "Insisto en que te vayas".

The opposite of queísmo (and possibly a hypercorrective reaction to it) is dequeísmo: the insertion of de before que where it would not appear in standard Spanish. It is less widespread than queísmo.