Hi there.
I'm watching 'One Strange Rock' series, and I've seen that Will Smith usually pronounces the article word "a" just like the name of the letter (witch, for a Portuguese speaker just like me, sounds as "ei" (I'm ignorant about phonetic symbols...)). However, in almost everywhere else, I hear people pronouncing it as it would sound in Portuguese, well, sounding as "a".
I mean, he says "a" in the way this letter sounds in "name", and not as it sounds in "car", witch is the way I've always heard people saying it.
Are there rules for the use of this (for me, totally) different ways of pronounce the article "a"? I mean, does he say this word in that way just to sound cooler, or is it a legit way to say that?
(I know that linguists hate the idea of "legit ways" to say things, but... uh... let this one pass, please.

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(BTW, thanks for the answers about the guinea pig thing. It cleared things, despite I still find it sounds a little absurd to call a lab mice or a fruit fly "guinea pig".

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