Sorry... my answer for (1) is somewhat different from アナスタシーヤ 1) so desu ka means "is that so?" and, just as the English phrase can be used in many ways, so too "so desu ka" can be used in many ways just by changing the intonation. So, it could mean "really, I didn't know that?" or "really, you gotta be kidding me" or "really, I don't believe that at all" to "that better not be true cause it makes me angry". You can only tell what the speaker means by the context of the conversation, his/her facial expression and his/her voice intonation. 2) "so desu ne" - just as アナスタシーヤ and Mario explained, this signifies agreement 3) "so desu" - unlike (1), this implies definite assertion. "it is so" 4) "da yo ne" - I'm a little confused by this one... "da yo" is masculine speech ("da yo" means "desu"), "ne" is more often used in feminine speech, so, as you have written it, this phrase is mix of both masculine and feminine speech. I'd have expected the phrase to be "so da yo na" which would be more consistently masculine and equivalent to "so desu ne" which is more feminine Of course, speech patterns change over time and these rules which were true 30-40 years ago may have transmogrified in the interim.

If you know their meanings, then you can use them as often as possible. In a normal conversation, we (non-Japanese) tend to limit ourselves when the other person is speaking not to interrupt what he/she is saying, we allow ourselves to say something, even if we want to agree, when the other person has finished their line of argument. You can also do the opposite, but if you do it too many times it is considered impolite. In Japan is quite the opposite. You can see in movies and anime that it is very common that while the other person is still talking and developing an idea the other person will say something like "そうですね" ("it really is like that, right?) or produce the sound usually represented by "うん・うん" (hum-hum), and the other person will not stop talking, will continue what he/she is saying, because in Japanese this is considered part of the interaction of the conversation, failing to do this, may, indeed, imply that you are less interested in the development of the conversation. I hope this helps.

well, according to my humble experience: 1) formal and polite way to show that u are not very interested in the topic. and when you hear it, u'd better round up and change the topic. that's de facto, de jura is "i am quite interested in what u r saying, please continue and give some more information about it" 2) u either agree to what the interlocutor is saying or show that u r digesting the material that u've just gained. or simply try to win some time to think about the response 3)answer in the affirmative 4) (just as) i've told. that's actually so! not sure that really all native speakers use them this way, but the number of my japanese friends who do is quite large.