Si (if and whether).

Si used for "whether" may be followed by a verb in any mood and tense as in English:

• No sé si habrá buena cosecha este año: I do not know whether there will be a good crop this year.
• No pudo, or supo, decirme si me entregaría los pagarés para la fecha convenida: He could not tell me whether he would be able to hand me the promissory notes for the date agreed upon.

Si used as the conditional "if," is followed by the verb in the present indicative or imperfect subjunctive, as:

• Si viene hoy le pagaré: If he comes to-day, I shall pay him.
• Si viene mañana le pagaré[2]: If he comes to-morrow I shall pay him.
• Le dije que si viniese le pagaría: I told him that if he came I should pay him.