Than - must be followed by "to" even if it has already been mentioned?

Question

I have such confusion with the word than, check out these examples:

I would prefer to have beaten him up than have been beaten up by him.

I prefer to die than kill someone

Should the preposition to be placed after than if I wanted to put the verb in the infinitive form? Like, I prefer to die than to kill? Or should it be like the first example? No preposition necessary?

And when talking about verbs ending in -ing: I would prefer doing this than doing that or I would prefer doing this than do that?

Answer 1

What you're describing is yet another example of ellipsis, in this case, where common prepositions are left out of sentences when they add no significant information. Either way is correct:

I would prefer to die than (to) kill someone.

Other examples:

She said she would prefer to go shopping than (to) see a movie.

I'd much prefer to stay home in bed than (to) go to work today.

If comparing two activities, then the most common construction is to make them both the same form. It doesn't really matter whether you pick the infinitive or the gerund:

I prefer skiing rather than surfing.

I prefer to ski rather than to surf.

However there are various other ways you can structure this kind of statement, that can be different depending on if you use the infinitive or the gerund:

I prefer skiing to surfing (and not "I prefer to ski to to surf")

I prefer to ski, not to surf (or "I prefer skiing not surfing)

Note: Your first example is somewhat unusual English. "Prefer" usually refers to things you would like to do, not things you would prefer to avoid. In this case "would rather" is more natural:

I would rather die than have to kill someone

She said she would rather make out with an orangutan than go on another date with him.

Answer 2

I prefer to die than kill someone.

The sentence is grammatical. You can also use "to" after than, but it's less common. Look at the following sentence:

I prefer to walk than/rather than to catch the train (The Free Dictionary).

As for the sentences:

I would prefer doing this than doing that.

I would doing this than do that.

You use 'than" to join infinitives, not two -ing forms or an -ing form and an infinitive. So the correct sentences are:

I would prefer doing this to doing that.

I would prefer doing this rather than that.

I would prfer to do this than (to) do that.

I would prefer to do this rather than that.