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.

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!