For a long time I avoided using forms such as lo cual in my speech, since they are somewhat formal and are found more frequently in writing. I usually made do with consecutive independent clauses instead: Se va a casar con mi mejor amigo. Eso me molesta mucho. This gets your point across, but the effect can be clunky.
The construction cosa que solves this problem. I have not seen this non-restrictive relative clause construction described in grammar books. It is somewhat informal and is more common in speech than in writing. Here’s how it works.
- Se va a casar con mi mejor amiga, cosa que me molesta mucho. She’s going to marry my best friend, which bothers me a lot.
- … cosa que no hubiera pasado si … … which would never have happened if …
Integrating cosa que into your Spanish can make you sound more natural. Your ideas will be connected more clearly and smoothly to one another. If you keep your eyes and ears open, you will notice a lot of opportunities to put cosa que into practice.