Verbs-which is the correct word

1. If he (was, were) over 18, he would have been hired.

2. Only one of the video clips (was, were) usable.

3. The typesetters, not the editor, (was, were) responsible for these errors.

4. Neither the manager nor the employees (was, were) aware of the policy change.

5. Both John and Steven (was, were) promoted.

6. The news from the rescue mission (is, are) encouraging.

7. Ten Steps to Greatness (has, have) been placed in the company library.

8. A child reminded me that the earth (rotates, rotated) on its axis.

9. Tim (don't, doesn't) ask for favors.

10. The president studied the page for a minute and (starts, started) asking questions.

Hi, Delly

Would you like to try first? Then you can be sure that we will do our best to help you. 

1. If he (was, were) over 18, he would have been hired.

You need were here, as it's a contrary-to-the-fact assertion (he wasn't over 18), esp in formal contexts.
I  mean were is better than was, but  still not the best (see further).

See:

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EXCEPTION   If I were ...
In the Present Unreal Conditional, the form "was" is not considered grammatically correct. In written English or in testing situations, you should always use "were." However, in everyday conversation, "was" is often used.

If he were French, he would live in Paris.
If she were rich, she would buy a yacht.
 
http://www.englishpage.com/conditional/presentconditional.html#presentunreal
(even if that applies best to present time, not to the past)

And strictly speaking, the were isn't the best choice for events occuring in the past time in contrary-to-the-fact situations, but it's tolerated. The best, IMO,  is the use of the past perfect:

1. If he had been over 18, he would have been hired.
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The Jungle by Sinclair, Upton

This was a critical time in Jurgis' life, and if he had been a weaker man he would have gone the way the rest did.

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The use of the past perfect here is justified  even on sequence-of-tenses reasons: he had to be over 18, before the decision to hire him would have been taken. The past perfect is the best in the opinion of many for showing such precedence.

I agree with Marius that, strictly speaking, it should 'If he had been over 18, he would have been hired'.