"It wasn't", in the third panel, seems like a simple enough statement in English, but a friend of mine wondered how best to express that in Mandarin, and ended up losing a whole night of sleep over it. He proposed bùshìle 不是了 ("it isn't anymore; it isn't any longer"), where le 了 indicates change of state: it has come to a state of no longer being. Google Translate renders bùshìle 不是了 as "not anymore", which is not bad.
Google yields 2,340,000 hits for "不是了", which would seem to indicate that it is an acceptable collocation. And this Yale website discusses sentences having shì 是 with change-of-state le 了 at the end under IIE, though not with bùshìle 不是了 as such. Change-of-state le with a negative is illustrated in section III, but again not with bùshìle 不是了 as such.
It is certainly acceptable to say xiànzài bùshìle 現在不是了 ("now it is not"), but that is not the same as saying "it wasn't").
I asked about a dozen native speakers their opinion about whether it would be possible to translate "It wasn't" in the third panel of the cartoon as bùshìle 不是了, and received some very strong reactions, of which I list a few here:
In this case, I would translate it as zài zhè zhīqián bùshì 在这之前不是 ("before this it wasn't"). I do not think there is a standard way to translate "it wasn't" since Chinese does not have tense. We have to say céngjīng bùshì 曾经不是 ("at some time in the past it wasn't"), guòqù bùshì 过去不是 ("in the past it wasn't"), yǐqián bùshì 以前不是 ("before it wasn't"), etc.
I would say běnlái bu shì de 本来不是的 ("originally it wasn't").
I think bùshì 不是 ("is not") is probably enough and most natural since Chinese relies heavily on context to distinguish tense. If you must make it clear that "it wasn't, but it is now" you probably can say something like yuánlái bu shì / běnlái bu shì, xiànzài shìle 原来不是 / 本来不是，现在是了 ("originally it wasn't, [but] now it is"). However, in my humble opinion, the latter is a paraphrase rather than translation, and sounds awkward. Or did I interpret the English wrong?
I think I will translate "it wasn't" into bù…shì… 不…是…. ("is… not…"), to express the feeling of the man. The ellipsis in bù…shì… 不…是… ("is… not…") is often used by netizens in China to express various kinds of feelings, such as helplessness / embarrassment / awkwardness, and so on.
I've never used bùshìle 不是了 before. But I have heard some Taiwanese say bùshìle la 不是了啦 ("it isn't anymore") before, especially young girls. I think bùshìle la 不是了啦 ("it isn't anymore") might be used to translate "it wasn't" here.
I would probably not use bùshìle 不是了 here, which would suggest that the tune used to be "Puff the Magic Dragon", but now it isn't any anymore.
Bùshìle 不是了 is incorrect in this context. Bùshìle 不是了 would be "not anymore" in English. The 了 points to the changed state, not the original condition / situation. Colloquially, however, it is completely proper (even preferred) to say bùshì la 不是啦 ("it wasn't"), which may sound like bùshìle 不是了, but is actually quite different. 啦 softens the tone and thus sounds more pleasant; but le 了 changes the meaning / implication.
The translation bùshìle 不是了 for the last panel that you gave is very wrong. Bùshì 不是 ("it wasn't") is the correct translation. Le 了 indicates a new situation that has come into being, rather than a simple mark of the past tense. Bùshìle 不是了 means "be not anymore" = bù zài shìle 不再是了 ("it no longer is; not anymore"). For example, Xiànzài de Měiguó zǒngtǒng shì Bush ma? — Yǐjīng bùshìle, shì Obama 现在的美国总统是BUSH吗？–已经不是了，是OBAMA ("Is Bush the president of America now? [No,] he no longer [is president], it's Obama").
I think bùshìle 不是了 means "it is not now but it was". It is present rather than past in terms of tense.
Such are the reactions occasioned when tense in English meets aspect in Chinese.
[Thanks to John Rohsenow, Xiaojue Wang, Maiheng Dietrich, Rebecca Fu, Cheng Fangyi, and Jing Wen]