Is this OK? But for missed the train, I wouldn't have had to take the bus.
I wanted to transform the following sentence by using 'but for' : If I had not missed the train, I wouldn't have had to take the bus.

If it's correct grammatically, then why should I use 'but for' instead of 'If... had' structure?

Question

Is this OK? But for missed the train, I wouldn't have had to take the bus.
I wanted to transform the following sentence by using 'but for' : If I had not missed the train, I wouldn't have had to take the bus.

If it's correct grammatically, then why should I use 'but for' instead of 'If... had' structure?

Answer

The expression "but for" expresses the idea of "if something hadn't existed or happened", but you need to use a noun or -ing form after it. In your example, it's " But for the missed train, I..."
FYI http://idioms.thefreedictionary.com/but+for

Answer

That's a strangely phrased question. If you want to change it, you can. You can choose one grammatically correct form, or another.

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