perquè/per tal que + Subjunctive

T'ho dic perquè ho sàpigues means I am telling you so that you know. The subordinate clause states the intended result of the main action, not a factual cause explaining it. That purpose meaning normally selects subjunctive.

Per tal que makes the intended result explicit in formal prose and also takes subjunctive. Keep the actor of that result clear: adaptem el text per tal que tothom l'entengui does not say that understanding has already occurred. A past main action commonly gives per tal que l'entenguessin. Avoid using an indicative form merely because the intended result is likely or because the main action itself is factual.