Learn Italian: Confido la tua supposizione sia valida.

In Italian, you can optionally drop the conjunction che before a subordinate clause, but generally only if the verb in that clause is in the subjunctive mood. Because confidare triggers the subjunctive (sia), you can say Confido la tua supposizione sia valida instead of Confido che la tua supposizione sia valida. This omission makes the sentence sound slightly more elegant and is very common in higher-level written or spoken Italian.

Why do we use the subjunctive sia instead of the indicative è here?

The verb confidare expresses a personal expectation, hope, or belief rather than a stated objective fact. Just like other verbs of hoping, believing, or fearing (such as sperare or credere), it requires the subjunctive mood in the following clause.

If I changed the main verb to so (I know), could I still drop che?

No. Verbs expressing absolute certainty, like sapere (to know), trigger the indicative mood, not the subjunctive (So che la tua supposizione è valida). Because the rule allowing you to omit che applies almost exclusively to subjunctive clauses, you cannot drop it when using the indicative.

How does confidare differ from sperare (to hope) or fidarsi (to trust)?