My suggestion is: forget about all the the different meanings. Learn them in real conversation and in a natural way. Don't worry if you make a mistake. You will learn from mistakes and then you'll gradually understand the differences. Or else you won't remember the definations anyway. Chinese people certainly won't look up a dictionary to see what are the differences. I think "language intuition" -- we call it 语感 in Chinese -- is more important. Unless, of course, if you are taking an exam, then you do have to remember them.

也 tends to mean that those things are equal, or some aspects (of things or people) are the same. But in the phrase "not only ... (but) also", the translation can be "不仅...也" ("不仅...还" is also right). 還(还) often expresses some comparative meaning similar to the word "even". e.g. 也 We need some food. We also need water. = We need both food and water. A: I'm afraid I'll fail the exam. B: Perhaps I'm also failed. 還(还) I hate homework. I also hate those teachers who assign me so much homework. Anyway, much of the time you can use either of them.

Most of time it means 也，but you should translate it according the contexts. like: I also like Belgian chocolate! 我也喜欢比利时的巧克力 We also need to buy more Belgian chocolate! 我们还需要买更多的巧克力！ And sisilia has already explained very clearly!

还和也有同一个意思:不仅仅而且.但又有不同,也有表示转折的意思, 还: ①表示现象继续存在或动作继续进行；仍旧：十年没见了，她～那么年轻丨半夜了，他～在工作。 ②表示在某种程度之上有所增加或在某个范围之外有所补充：今天比昨天～冷丨改完作业，～要备课。 ③用在形容词前，表示程度上勉强过得去（一般是往好的方面说）：屋子不大，收拾得倒～干净。 ④用在上半句话里，表示陪衬，下半句进而推论，多用反问的语气；尚且：你～搬不动，何况我呢？ ⑤表示对某件事物，没想到 6表示早已如此：～在几年以前，我们就研究过这个方案。 yě]副词。 ①表示同样：水库可以灌溉、发电，可以养鱼。 ②叠用，强调两事并列或对待：他～会种地，～会打铁│游客里面～有坐车的，～有步行的。 ③叠用，表示无论这样或那样；不以某种情形为条件：你去我～去，你不去我～去│他左想～不是，右想～不是。 ④表示转折或让步（常跟上文的‘虽然、即使’等呼应）：我虽然没见过，～听人说过│即使你不说，我～知道。 ⑤表示委婉：倒～罢了│～只好如此。 ⑥表示强调（常跟上文的‘连’字呼应）：连爷爷～乐得合不拢嘴。 好了,累坏我了,从手机词典拷过来,本来想自己解释的,发现太多了说不清楚