To introduce a clause with [preposition] + what in Spanish, use the following:
[con, para, en, de, por ...] + lo que + [clause]
Read and listen to these examples:
En lo que a mí respecta, no tengo más preguntas.As far as I am concerned, I don't have any more questions.
Ten cuidado con lo que deseas.Be careful what you wish for.
Para lo que vale la pena, es mejor no enfadarse.For what it's worth, it is better not to get angry.
No sé de lo que hablas.I don't know what you are talking about.
Se enfadó por lo que le dije el otro día.She became cross because of what I said to her the other day.
You can never place the preposition at the end of the clause like in English:
Ten cuidado lo que deseas con. / Ten cuidado con lo que deseas.
Be careful what you wish for.
without a preposition expresses what/which
when referring to a whole previous idea or sentence. See Using lo que = what, which, that which, the thing that (relative pronouns)
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