Would have to is here just the past tense of Will have to
You’re right: this is the sense of would which the OED mentions is used for a past-tense version of will. It’s used in reported speech as a “future in the past” as it is sometimes called.
The (paywalled) OED3 entry that applies here is specifically
- In indirect reported speech or thought, or its virtual equivalent, in
statements in the second and third persons reporting an original statement
(esp. of intention) in the first person: intended to, meant to; was going to.
Recent citations provided for that sense 21 include:
- 1967 A. Wilson No Laughing Matter ɪɪ. 153
Shouting as he went that he would bloody well never return.
- 2018 A. Stein Unbound Afterword 284
In August 2017, Trump announced that he would reverse a policy that enabled transgender individuals to serve openly in the military.
But don’t read too much into their reference to specific grammatical
persons (first, second, third), because most native speakers today use
will/would for all persons.
So simple sentences like these in the present tense:
- Today’s forecast says it will rain tomorrow.
- The weatherman says it is going to rain tomorrow.
Merely become these corresponding versions in the past tense under the backshifting needed for reported speech:
- Yesterday’s forecast said it would rain today.
- Yesterday the weatherman said it was going to rain today.
Related ELU questions include 1, 2, 3.
The choices of Collins
I can see why you’re a bit puzzled over that Collins translation of the English exclamation into Spanish:
- It would have to rain today, of all days!
The answer to your question about this particular use of would have to is actually simpler than it looks, but it takes seeing how they translated that into Spanish for that to become crystal clear: they used a past tense in their translation into Spanish, not anything subjunctive or conditional or such. That’s the key insight here.
Notice how for their translation into Spanish, they gave:
- ¡Tenía que llover hoy justamente!
Which I might have translated as this in English:
- It just had to rain today!
So as you see, there’s some exasperation in the exclamation.
There’s a seeming mismatch in aspect between the English and the Spanish, but that’s always going to happen because English cannot distinguish between the Spanish imperfect tenía que and the Spanish preterite tuvo que. Don’t worry about that bit. Their choice of tenía que still tells you everything you need to know about this would have to of theirs: it’s actually a past-tense use!
So what’s happening is that this is the would have to that is merely the past-tense or “back-shifted” version of the modal verb will have to which is commonly used for the future. This is therefore the “future in the past” sort of would. It’s not anything fancy.
Mostly for Spanish speakers
Because you started with an English-to-Spanish translation, I’m going to assume you’re a native—or fluent, or at least capable—speaker of Spanish. So here are three specific citations of tenía que llover hoy in Spanish with which I’ll include a bit of surrounding context so that maybe you will better understand the sense of English would have to here as Collins has used it. I’ve placed the sentence that the Spanish phrase occurs in (or nearly occurs in) in bold:
First Spanish example
Rubén Cervantes fue por su cartera, de la cual extrajo la imagen de un santo.
―Aquí traigo este San Juditas pero no me ha servido de nada. ¿Qué estoy
haciendo mal? No ha llovido en todo el año pero tenía que llover hoy que
me puse mis tenis blancos. No doy una.
―Vamos a La Ballena ―dije―. Ahí me sigues contando.
Second Spanish example
―¿Por qué tenía que llover hoy? ―dijo―. ¿Por qué no pudo llover
Su padre, tirado en ese momento en el brazo del sofá con el periódico de
gobierno, hizo sonar las hojas con disgusto:
―Porque es simplemente lo que sucede, eso es todo. La lluvia hace que el
―¿Por qué, papá?
―Porque es así.
Third Spanish example
Segundo, me prepararon para el día en que estuviera en un parque de
diversiones y uno de mis hijos comenzara a llorar porque aparentemente el
día se había arruinado.
―¿Pero por qué mami, por qué tenía que llover hoy? ―Esa era su
pregunta una y otra vez―.
―Bueno, estamos en primavera, en la Florida. Y en primavera, siempre
llueve. La lluvia es necesaria para que todo se renueve, para que los ríos
crezcan, para que tengamos cosechas.
Loosely translating just the bold sentences from those three examples into English, I might choose to write them this way:
“It hasn't rained all year but it just had to rain today, the day I put on my white tennies.”
”Why did it have to rain today?” he said. “Why couldn’t it rain tomorrow?”
”But why, mommy? Why did it have to rain today?”
In all three cases, I managed to use a translation into English without using the would have to that Collins used. I did that not because Collins is wrong, but because I wanted to show you other ways to think of it in English, ways which I hope will shine more light on this use of would have to of theirs.
This would have to rain is simply a past-tense of will have to rain, as the Spanish versions all illustrate more clearly. It is definitely not any sort of “conditional” type of would have to rain because if it had been, they would have chosen a “fancier” form of the verb in Spanish than their simple tenía que llover — perhaps something like tendría que llover or even habría tenido que llover. The first of those two is easy to think of examples for, the second a bit harder just because compound tenses are always going to used less frequently than simple tenses are. These are all conditional-type uses involving counterfactuals, and that’s not what Collins chose for your example:
How much would it have to rain to fill the reservoir?
¿Cuánto tendría que llover para que se llene el embalse?
It would have to rain a whole week at this rate before we had to start worrying.
Tendría que llover toda una semana a este ritmo antes de que tuviéramos que empezar a preocuparnos.
It would have had to rain a whole week at this rate before we would have had to start worrying.
Habría tenido que llover toda una semana a este ritmo antes de que tuviéramos que empezar a preocuparnos.
As you see, your original It would have to rain today! exclamation just means that it was going to rain today in the past tense the way it is in citations 1, 2, and 3. It’s not any kind of fancy conditional use like it is in my off-the-cough examples 4, 5, and 6.