You are the only person in whom I can confide.
Tu eres la única persona a quien puedo confiar.
—RAMOS CARRIÓN.
(a) The only thing (fact) is rendered lo único.
The only thing she knew was that he had taken that train.
Lo único que sabía era que había tomado aquel tren.
—BLASCO IBÁÑEZ.
(b) As an adverb only is rendered sólo, solamente, or, modifying expressions of quantity, no más ... que (de before numbers). For the order of the phrase in the sentence, see the examples.
If I only had what my father gave me, I shouldn't have enough to pay the baker.
Si sólo tuviera lo que me da el padre, no tendría para pagar al panadero.