Subjuntivo en la cocina: para que

Rule: we express purpose with "para que" + Subjuntivo when the subjects are different: "tú tapas" — "el caldo se concentre". The cooking context here is no coincidence: in Spain, food is a social event, and dishes (for example, paella) are often cooked "for everyone", which gives great real-life examples for language practice.

Here there are two different subjects: "tú cierras" (the action) — "la salsa no salpique" (the purpose/result). With different subjects we use "para que" + Subjuntivo.

A fixed everyday phrase: "Te lo digo para que lo sepas" = "I’m telling you so that you know / so you’re aware." Shared cooking and daily life in Spain are often linked to chatting around food, so expressions like this sound especially natural in that context.

You need the Subjunctive: "no se oxide". This is how we express PURPOSE: "we add lemon so that the fish doesn’t oxidize."

Here there is one and the same "doer": tú pruebas and tú "saber". So we use "para" + infinitive: "para saber". Soups and sauces are a typical context in Spanish cuisine (including cold soups like gazpacho), which makes them very handy for examples like this.

"Que yo sepa" is a fixed expression with the Subjunctive meaning "as far as I know". Spaniards can be very strict about the "authenticity" of regional dishes, so this kind of caveat sounds very natural when talking about paella.