Why use "would have to" in the past tense?
Here I'm not sure "would" is being used to create a conditional mood. I mean, technically it might, but it's not so clear to me. A conditional mood created by "would" or "would have to" would be something like:
- If you went to the party, I would have to go.
I knew that I would have to be in great physical shape if I wanted to win the race.
(Here we have "would have to" referring to the past)
See these Google Books results for:
"I knew that I would have to"
... if you're interested in this conditional structure referring to the past.
As I said, I'm not sure that in your example the "would have to" is creating a conditional mood.
Let's just say you confiscate someone's mobile/cell phone in order to force them to use the landline phone. I don't know any simpler way to say this using "so that" than to say:
- I confiscated his phone so that he would have to use the landline.
I don't see any conditional here, unless you read the ellipsis of ("if he wanted to make a call") at the end of the sentence. To me it just sounds like another way of saying:
- I confiscated his phone so that he would be forced to use the landline.
(a longer way of saying it).
So why do we use "would" here? Well, I'm not sure.
- I confiscated his phone so that he has to use the landline. (sounds strange to me).
Here are some more examples from Google Books for so that I would:
- I wrote my name on the book so that I would not lose it.
- So clearly, his intent was to hit me hard so that I would drop the ball.
- I packed my case so that I would be ready to go at the last minute and I called my best friend Ellie...
and "so that he would have to"
- Then she shut the door, leaning against it so that he would have to push her out of the way to get back outside.
I'm not sure if these examples are conditionals. Also, I don't think I see these uses of "would" defined in the dictionaries either, unless we read these as "so that he/she might".