Pitman's Commercial Spanish Grammar (2nd ed.)

The English verbs "shall," "will," "should," "would," "may," "might," when used as auxiliary verbs (viz., as mere signs of the future tense, conditional or subjunctive moods respectively) are rendered by the corresponding terminations of the Spanish verb[195] --

I shall go: Iré.
Thou wilt go: Irás.
We should go: Iríamos.
You would go: Vs. irían.
I hope he may succeed: Deseo que tenga buen éxito.
I wished he might come: Deseaba que viniese.

Footnote 195: "Shall," "should," "will," "would," are also rendered by the Subjunctive Mood when according to the Spanish rules the verb should be Subjunctive. Example --

It is (was) possible that he will (would) have to work late:
Es (era) posible que tenga (tuviese) que trabajar tarde.

When they are used as principal verbs they are translated by Deber (shall, should), Querer (will and would), and Poder (may and might), as --

You shall go: V. debe ir.
You may speak: V. puede hablar.
I will not buy more: No quiero comprar más.
You should accept our terms: Vs. deberían[196] aceptar nuestras condiciones.

We would willingly see that done: Quisiéramos mucho verlo hacer.

Footnote 196: The Imperfect Indicative debía, podía, quería, is often used for "debería," "podría," and "querría." This occurs also with other verbs, colloquially: Yo se lo daba si estuviese aquí for Yo selo daría si estuviese aquí.

"Can" is translated by the pres. or future of Poder --

I can do it now: Puedo hacerlo ahora.
I can do it next month: Puedo or Podré hacerlo el mes próximo.

"Could" is translated by the imperfect indicative or the conditional mood of Poder --

I did it whenever I could: Lo hacía siempre que podía.
I could do it if I had the necessary means: Podría hacerlo si tuviese los medics necesarios.

"Would," meaning "used to," is, of course, translated by the imperfect indicative --

When he was in Paris he would sit in the cafés for hours: Cuando estaba en Paris se pasaba horas enteras en los cafés.

"Should he do," etc., is translated "if he should do," etc. (si lo hiciese).[197]

"If I were to go" is translated "if I went" (si fuese).

Footnote 197: After si (if -- Conditional) the verb is present indicative or imperfect subjunctive (see Lesson XXIV).

"Can" is translated by saber when the faculty expressed is the result of learning, as --

Can you play the piano? ¿Sabe V. tocar el pianoforte?

"To be to," "to have to" are translated by Tener que, Haber de or Deber (see Lesson XXVII), as --

Who is to write out that invoice? ¿Quién ha de (debe) escribir aquella factura?
I have to be very careful: He de ser muy cuidadoso.
I shall have to work hard: Tendró que trabajar fuerte.

"Let" as a principal verb is translated by dejar or permitir; as an auxiliary it corresponds to the Spanish imperative mood --

Let him speak: Déjele V. hablar, or Permítale V. que hable. Let him ask for it and we shall give it to him: Pídalo (or que lo pida) y se lo daremos.

"To let (or hire) a house" -- "alquilar (una casa)" -- I have let my house: He alquilado mi casa.