"would have" vs. "had" vs. "should" vs. "has" etc

Question

Please consider the following line:

What can we do? We need an alternative. It * * * incorporate X and * * * offer Y.

Which of the following verbs and forms is correct?

1. has to
2. needs to
3. must
4. had to
5. should
6. would have to

Which is best? Note that this is a scientific text.

How do they differ in meaning? The intention is that 'the alternative' does not yet exist and will be hard to find, so it looks conditional-ish to me. I have the feeling that 6. is the best, but it is so verbose. Does another form express the same?

Answer 1

Firstly, I think 5 of the 6 listed options are grammatically correct to replace the * * * gaps in your quote. Only had to would look odd to me, as we're not talking in the past tense.

I don't see any grammatical issue with using any combination of the 5 valid options in either gap, but in my experience it would be most common usage to just place one of these options into the first gap only (referring to X), as the same will then be implied for Y as well:

What can we do? We need an alternative. It must incorporate X and must offer Y.

would usually be written or said as:

What can we do? We need an alternative. It must incorporate X and offer Y.

In the second quote above, it is implied that the alternative must offer Y.

I terms of meaning, I would say the following are all synonymous:

1. has to
2. needs to
3. must

They all imply that the property being referred to (X or Y) is mandatory, so the alternative being considered is not an option unless it has those properties.

The option

5. should

is much softer, with no direct implication that the requirement is mandatory, so it leaves open the possibility that viable alternatives could be found which do not incorporate X or offer Y.

I agree with you in your assessment that the best option is:

6. would have to

This retains the mandatory nature of the first 3 options, but also has the connotation that you describe, that it might not be easy to find such an alternative. You could just as appropriately use would need to. It's the presence of the word would which makes these options sound more conditional.

So I would go with:

What can we do? We need an alternative. It would have to incorporate X and offer Y.

You can augment the questionability of the likelihood of finding such an alternative with something like:

What can we do? We need an alternative. It would have to incorporate X and also offer Y.

or

What can we do? We need an alternative. It would have to incorporate X while also also offer Y.

The latter option here implies that there is some conflict between being able to incorporate X while simultaneously offering Y.