(I've noticed that some native-speakers of American English realize this, and accommodate our foibles by enunciating these words very clearly and even in some cases modifying the vowel sound. )
*PS to clarify: When I said there were two distinct vowel sounds, I was over-simplifying. In most cases, when not emphasized (for example, because of contrastive stress) the Br Eng pronunciation of 'I can' is /aɪ kǝn/, so there are really three. A Br English speaker, hearing anything but a schwa - when there's no other reason to suspect emphasis - assumes a negative. For example, if I said to my wife (also a native speaker) /aɪ kæn si:/, she would want to know a why I couldn't see, and b why I was using that silly accent. So beware...
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