"An t-orainsear dona."
Translation:The bad orange.
The orange is bad -- would be -- Tha an t-orainsear dona.
An t-orainsear dona -- can only be -- The bad orange.
If "tha" doesn't appear, then the English sentence doesn't include "is".
(This applies to "tha/chan eil/a bheil" sentences. Later in the course, "Is e ..." type sentences will frequently have a translation that includes "is". For example, " 'S e an t-orainsear dona a th' ann.")
I had wondered the same, and I'm not certain exactly, but I notice that B,F,M,P are all sounds made with the lips touching. F is kinda of an outlier because sometimes people make it by touching the teeth to the lower lip. Either way, it's made with the mouth closed, so I'm guessing that could be what these all have in common.
Also the English sounds for V,W, and sometimes U get made with the mouth closed. But V in Gaelic would always be spelled BH or MH (DH? Not sure) so that's accounted for. I don't think W exists in Gaidhlig. And we've already seen (with rain) that the U in "uisge" gets a special solution ("t-").