2 questions, 还 \ 也 , and 了

hi. i have a couple of questions.

1. could someone please tell me the difference in use of 还 and 也 ?

2. also im wondering how to use 了 (change of state) in the sentences that talk about the future. one example

这个星期天气都不好，下个星期天气才会好了

do i need the 了 in this sentence, because its a change of state, or dont i use 了 like that while talking about the future.

please use an example if you have a little bit of time, so i can understand the differences easilier. thanks