か只有一个读音，那就是ka。 只不过，在发ka音的时候，嘴巴不能有太多的“气”出来，所以听起来像ga。你可以把自己的手掌放在嘴前，感受发ka 和ga音时 气流的变化，就知道了。 は做助词时念wa ，比如自我介绍“私ha。。。” 还有问候语こんにちは 其他情况念ha。比如，明信片叫 はがき hagaki

I agree with j0nasan

1.对于初学者来说都ka 就可以了，这样便于记住正确的词汇。 其实在句末，或词末的时候，这个音，应该介于ka和ga之间的。 2.一般是做助词的时候才读wa

I have a feeling the first half of the question is about the perennial problem of phonemic distinctions being made on different axes. In Japanese it's always between voiced and unvoiced, not between aspirated and unaspirated as in the case of mandarin Chinese. Some allophones of the *same* phoneme in Japanese are actually aspirated while others are unaspirated, and this creates confusion among speakers of Chinese, especially if they only speak northern verities...

か is always KA Sometimes が on the other hand can change sound. It can be pronounced both GA or NGA depending on level of formality and position within the sentence.