isn't 又 used in the past tense? I can't understand the meaning here 该，了，又. I found two more sentences that are unclear to me: 1) 她是不是又在发神经了 2) 她又发神经了 which shows 在 in the first sentence ？
So 又 here only means again, it's not really associated with past tense or present tense sentences. (Do not understand it in terms of tenses, you'll confuse urself). You can use it in various ways:
她又在干...: she's doing smth again.
我又不舒服了： I'm not feeling well again.
怎么又发脾气了？：why is smbd getting mad again?
明天，她又要说我了：tomorrow, she'll criticize me again.
昨天晚上，还不是又打球去了：yesterday night, we went to play basketball again.
See, it can be used with almost any tense, so don't understand it like so! 又 is usually followed by some action, as exemplified above.
再 is different, when used, it's usually attached with a single character verb (in this context), for instance: 再见，再嫁，再写 etc. You can't say 再不舒服 (uncommon and weird).
I was originally trying to go intuitive, but in response to @Tatiana 's question, let's get a bit grammatical. The general rule is that "又”must be used in actions that have happened or is happening, “再” is used in actions that is going to happen.
Let's use a few examples from 《学汉语》
- "你刚走怎么又来了？我现在没时间，你明天再来吧。"; “又”means the verb “来” is happening atm.
- "我要给在朋友写信。前天我写了三封，昨天晚上又写了三封，今天晚上我还得再写两封。"; “又” means the verb “写信” has happened before.
In both examples, “再” means some action is going to happen some day.
The difference in the "again" should be decided based on context most of the time. Let's use another example and its explanations (from the same book):
“再坐会儿” is what 吴明 already said，why does the "husband" not say “又” but instead say“再”呢？Because “再坐会儿” is the “husband” repeating what 吴明 said to the "wife"，so we must use“再”，not“又”。This can be seen as the “过去将来时” (past future tense)。
一會ㄦ媽媽又該生氣了 - Mother should(該) get angry again(又) in a short moment.
- Note that, here, "了" is a particle that indicates the change of state. The adverb "again(又) suggests "get angry" is an expected reaction of the mother (as it happened before) after the provocative action.
The two sentences below essentially are the same except differing in "tense":
她是不是又在发神经了? - Whether or not her nerve is losing control again?
她是不是又发神经了? - Whether or not her nerve loses control again?
- Note that, in both sentences, "了" again is a particle that indicates the change of state. The adverb "again(又)" indicates her loss of control is a recurrent event that has happened before.
又要/又该/又得 xxx 了 are kind of exchangeable, meaning the following will happen (again), if the mentioned precondition happens (again).
又在 xxx 了 means "already started doing".
for these words, in some common usages, you may assume that the speaker doesn't like to see it (again), maybe even wants to prevent it from happening twice in prior. but it's also possibly just a plain statement "他又在学习了，还是不打扰他了"
in less cases, when the subject is "I", it may sound like some kind of showing off or self-deprecating.