fail

[English4everyone]

As far as I know, we don't use any prepositions for "fail", for example: He failed his final exam. But I saw some sentences with "in", for example: But his brother failed in mathematics.
I want to know "fail" and "fail in" are completely the same and always interchangeable?

[bhaisahab]

As far as I know, we don't use any prepositions for "fail", for example: He failed his final exam. But I saw some sentences with "in", for example: But his brother failed in mathematics.
I want to know "fail" and "fail in" are completely the same and always interchangeable?

No, they are not always interchangeable.

[emsr2d2]

We wouldn't use "He failed in mathematics". We might use:

He failed maths.
He failed his maths exam.

At a push, you might hear "He failed at maths".

[English4everyone]

So you mean "fail" is never used with "in"? If there are any examples, could you give me?
Or maybe it's another difference between British and American English.

[BobK]

So you mean "fail" is never used with "in"? ...

Nobody said that.

b

[emsr2d2]

So you mean "fail" is never used with "in"? If there are any examples, could you give me?
Or maybe it's another difference between British and American English.

I didn't say it's never used with "in". I simply gave you the natural BrE version of your specific sentence. "Fail" can certainly be used with "in":

He failed in his attempt to swim across the English Channel.
They failed in their attempts to drive me out of the company.

[Chicken Sandwich]

NOT A TEACHER

But his brother failed in mathematics.

This to me means that his brother failed as a mathematician. It doesn't mean that he flunked math.