Going to Russia

Hi Rotter

Yes, the possibility of making a new drug always exists, but the degree of probability varies.

In sentences 4 and 5, the degree of probability that a powerful new drug to kill the AIDS virus will be developed is identical. You presented the condition (in the IF-clause) as likely in both sentences.  It is the result that is presented differently.  The most likely combination is sentence 4.

However, there is a good argument for a "mixed conditional" here in that it is well-known that the cost of AIDS medicine is frequently not affordable for AIDS patients.  Therefore possibly only a limited number of patients might actually benefit from the new drug -- and thus the use of would rather than will might be justified for that reason. 

If the development of the ultimate AIDS cure is seen more as theoretically possible rather than quite likely (as in your sentences 4 and 5), then you should begin the sentence this way:

If researchers produced a powerful drug to kill the AIDS virus, ...
-OR-
If researchers were to produce a powerful drug to kill the AIDS virus, ...

(And the sentence would end the same as 5.)

I fully agree with Yankee on 4, 5.

Whenever you see "would," this should tip you that the situation is really imaginary/hypothetical/optional.

"Will" inserts more confidence about the future situation. The indicative is then usually used in the if clause.

Pavarotti, widely regarded as one of the world's greatest opera singers, has been dogged by ill health in recent years.

Before the cancer was found, he cancelled performances as a result of neck and back operations, the latter of which was complicated by an infection, and laryngitis.

After the surgery on his pancreas last year, his manager said the tumour was "completely removed".

But despite hopes that he would be able to resume his farewell tour this year, his health has not allowed him to return to the stage.

Before this latest stay in hospital, however, his manager said he was "feeling stronger" and recording an album of classical religious music as well as teaching students.

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Please look at the third sentence of the above.
But despit hopes that he would be able to resume his farewell ...

I belive the reason to use 'would be' is the imaginary nature.
However, it is not wrong to use the verb 'will' here.

England and Germany will play play a football match.

English fans could say ' We hope we will win.'

My point is that it is not incorrect to use the 'will' form when it comes to hope.
I would like to read your comment.

Thanks Grammar Geek
In the sentence you will read the words 'his has not allowed him to return to the stage'.
The above means present perfect tense.

1.I have allowed you to answer my questions.
2.I have not allowed you to answer my questions.
For me both of the above sentences are present perfect ones.

The sentence says this year. It means 2007 and has not allowed is suitable here.
I don't know why you classify it as a past event.