Se sabe a ciencia cierta que el calentamiento global lo están causando los humanos.It's known for certain that global warming is being caused by humans.
No se sabe a ciencia cierta si Maria Antonieta en realidad dijo, "que coman pastel", pero es una creencia popular.It's not known for sure if Marie Antoinette really said, "Let them eat cake", but it's a popular belief.
No sabemos a ciencia cierta si Netflix comenzará a incorporar anuncios a su servicio. Hasta ahora, lo han negado.We don't know for a fact if Netflix will start adding commercials to its service. So far, they've denied it.