Part 2 (I ran out of space)

Why "it weren't" and not "it wasn't"?

In Spanish, we wouldn't say "si no fue por las ideas de Sara.' Instead, we say "si no fuera por las ideas de Sara".

In English, it's the same. We don't say "if it wasn't for Sarah's ideas." Instead, we should say "if it weren't for Sarah's ideas."

Note: there is disagreement about this in academic circles. Some people do think it's OK to say "if it wasn't for..." but it generally sounds better and more formal to say "if it weren't for".

"If it hadn't been for Sarah's ideas" would also be a correct way of writing the sentence. Your way is also correct.

However, in this context, I think that "weren't" is being used because it's the subjunctive.

I was waiting to see if somebody more knowledgable on this topic commented instead. However, no other native speakers have commented, so I will tell you my opinion (it might be wrong).

When we use the subjunctive in English, we change the verb. Singular verbs become plural verbs.

Example of indicative mood: "I was a little girl." (Yo era / fui una niña pequeña).

Example of subjunctive mood: "If I were a little girl"  (Si yo fuera una niña pequeña).

We often use the subjunctive when giving advice. We say "if I were you..." (this is the English equivalent of saying "yo en tu lugar").

Example of indicative: "I am not you." (No soy tú).

Example of subjunctive: "However, if I were you, I would do something."  (Sin embargo, si yo fuera tú, haría algo.")

"If it hadn't been for Sarah's ideas, we wouldn't have been able to...."  means "si no hubiera sido por las ideas de Sara, no hubiéramos podido...." 

"If it weren't for Sarah's ideas, we wouldn't have been able to..." means "si no fuera por las ideas de Sara, no hubiéramos podido...'

I also ran out of space!

Some examples of the sentence types above:

1st present/future likely:  If you make a snowman, it will melt, (it's a warm day at the end of winter in a cold climate)

2nd present/future unlikely  If you made a snowman, it would melt. (we live in the desert)

2nd present future imaginary: If I were a snowman, I'd be concerned about global warming.

3rd past imaginary: If you'd made a snowman when you visited the desert last week, it would have melted.

There are of course all sorts of mixed conditionals, and even these above can be further complicated by substituting words like may, might, can, could, when etc .

I would say Elizabeth's ideas are spot on regarding the subjunctive and the disagreement regarding was vs were. Same goes for the possibility to use the more textbook construction "If it hadn't been for Sarah's ideas, we wouldn't have been able to..."

In terms of which conditional, I find it helpful to ask the following questions regarding the meaning of the sentence:

1. Does it describe something in the past or present/future? (present/future= 1st or 2nd,  past = 3rd)

2. Does it describe a possible/ likely,  possible/unlikely or imaginary situation? (possible/likely = 1st, possible/unlikely = 2nd, imaginary present =2nd, imaginary past=3rd)

Clearly the events in the sentence mentioned are in the past (wouldn't have been) and imaginary, i.e. Sarah did have her good idea, and the fashion show was a success. Therefore we can think about it as a kind of third conditional sentence where the idiom 'if it weren't for' replaces if + past perfect.

This construction can also be used in a mixed conditional. Compare

If it weren't for cold weather, the snowman would have melted. (past)   with     If it weren't for cold weather, the snowman wouldn't be here. (present)

Exam tip: while your answer was grammatically correct, I'd say the presence of the word "FOR" in bold would indicate that they were looking for something different. I wonder if a sentence with a similar idiom "If not for" would be accepted, as in

If not for so many exceptions, the conditionals would be easier to understand:)