Q:

Can I say "in a same way"?

I've been told that the article for 'same' is always 'the', but sometimes I think it feels right to say 'in a same way' when I have not yet but am going to talk about the way or when I won't talk about it at all because that is not important.
For example, I just said (at another place)

A and B are used in the same way, A+ and B+.

but could I say it with 'a' instead of 'the'?

A and B are used in a same way, A+ and B+.

A:

No, you can't use "a" and "the" in the same way here. "The same way" need not refer to any way that has already been expressed, but can be used to refer forward to a way you're about to explain… or even simply imply a way from context.
Fundamentally, the problem is that using "a" breaks the idiomatic "the same". "A same" makes no sense, and "a way that is the same", while grammatical, is quite clunky. So instead, the definite article takes over for both and it's merged to become "the same way".

