I think it would be helpful via the following sentences :
1、Would you like anything else to eat? 你想要其它吃的吗？ under the circumstance, we usually add the word "还“,rather than "仍然”， in the front of the word ”想“. so it is changed into this 你还想要其它吃的吗？
2、He is very young as before.他仍然很年轻。in this case,we only use 仍然 to express. If we use 还 instant of 仍然， it means He is so young that he can live for a long time.
hope these idea will work , good luck