Here's a link in English to supplement those provided above about the difference in "g" sound.
Japanese phonology - Wikipedia, the free encyclopedia You'll need a bit of understanding of linguistics terminology, but it's not too complicated. Just think of the two sounds as those of "give" and the "ng" in "sing." Native English speakers should already be able to hear the difference (because we have the difference in English already!) if they know to listen for it.
What's interesting is that, according to the phonology research, there are actually
three "g" sounds in Japanese, represented in the international phonetic alphabet by /g/ (hard), /ŋ/ (soft like in "sing"), and /ɣ/ (we don't have this sound in English, but it exists in Dutch, Greek, Irish, Polish, etc.). To me, it's like making the soft sound, but adding a bit of Spanish's "j" sound a bit. You can hear it at
Voiced velar fricative - Wikipedia, the free encyclopedia.
But don't worry about the third sound. It's used by a minority of speakers.
In my opinion, the useful and simple rule (but only beginners should rely on the rule rather than experience) is: hard-g for the beginning of words and soft-g for the particle. This is, of course, nowhere near the actual truth of the matter, but it's a useful start for beginners.
You can also get into the middle of words that are loaned from Chinese, etc., but that comes more easily through experience than memorization of rules.
For a similar phonological issue, read about yotsugana:
Yotsugana - Wikipedia, the free encyclopedia. It really shows the variety of Japanese.