In this paper, by using Gr\"obner-Shirshov bases for Rota-Baxter algebras, we
prove that every dendriform dialgebra over a field of characteristic 0 can be
embedded into its universal enveloping Rota-Baxter algebra of weight 0.
In this paper, by using Gr\"{o}bner-Shirshov bases, we show that in the
following classes, each (resp. countably generated) algebra can be embedded
into a simple (resp. two-generated) algebra: associative differential algebras,
associative $\Omega$-algebras, associative $\lambda$-differential algebras.