I should pronunciate "is" instead "iz", like "he is good" , I may be wrong. Tell me what kind of situation, I should pronunciate "is", not "iz", thank u.
how to pronunce "is" before a word that begins w/a
In English dialects in general, "is" is /Iz/. There are dialects which will realize "is" as ending with [s] in many/most cases, but this is due to final devoicing and there are other phonetic cues present (particularly vowel length) indicating that the underlying phoneme is still /z/. Note that General American and Received Pronunciation are not generally considered as having final devoicing, but they are generally considered as having weaker voicing of final lenis (aka "voiced") obstruent phonemes than normal (I have my doubts as to whether their final lenis obstruents are necessarily truly voiced at all in all cases, though).
In these dialects with final devoicing (or at least those in the Upper Midwest), [s] or, more conservatively, [z_0] is realized unless the next word begins with a vowel, semivowel, or sometimes a lenisobstruent, where then [z] is very likely to be realized instead of [s] or [z_0]; such is not fixed, though, as [s] may still be realized at times in such cases.
(For more information on the transcription system that I am using, which is known as X-SAMPA, go to http://en.wikipedia.org/wiki/X-SAMPA .)
In these dialects with final devoicing (or at least those in the Upper Midwest), [s] or, more conservatively, [z_0] is realized unless the next word begins with a vowel, semivowel, or sometimes a lenisobstruent, where then [z] is very likely to be realized instead of [s] or [z_0]; such is not fixed, though, as [s] may still be realized at times in such cases.
(For more information on the transcription system that I am using, which is known as X-SAMPA, go to http://en.wikipedia.org/wiki/X-SAMPA .)
In my speech, and - as Travis indicates - in most English dialects, "is" is always pronounced [Iz] or [@z].