Hmm. That's true that those examples use indefinite articles but I think those nouns become countable in that particular case. That is to say that I think you can count them up to one and no further. For example:
(1) He was inspired with a new courage.
This isn't courage in general but rather it's the courage that 'he' was inspired with at that moment. It's one courage that isn't specific so you use the indefinite article. However, you can't have more than one courage. For example: you would say "he was inspired with the courage of ten men" not "the courages".
(2) When he finished speaking, there was a long silence.
This is the same. You could say "there was silence" but as you said the adjective triggers the indefinite article because this is making the silence more specific. It's a 'long silence' as opposed to a short one.
(3) A heavy rain began to fall.
Again, it's a heavy rain as opposed to a light rain etc.
That said, I don't think you can use "a" with "good English" because good English is a little more general. It just doesn't sound quite right to use the indefinite article there. I think it might be that English covers a lot of different aspects like speaking, writing, reading etc. so you need to refer to it in a general sense.
Anyone else have any ideas?
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