First of all, the full traditional form for 干 (dry) is not 乾. This is "chien" as in Chien Lung Emperor of the Ch'ing Dynasty or as in 乾坤 (Heaven & Earth). The difference is that instead of 十 in the bottom of left hand side component, it has the 干 component that is 亁 (you have to magnify this character to see that extra stroke!). Can you see the difference? Even many native speakers make this mistake and so in time, they became synonymous and only from the context, can they be told apart. Another variant is 乹. 榦 and 幹 on the other hand are variant forms (異体字) of 干 (dry) used mainly in calligraphy. Even 干 can mean trunk, pole can be written as 桿 . 旱 = drought. This can be like in English variant spellings of tire vs tyre, storey vs story, grey vs gray. So languages have their own peculiarities due to historical context etc. You just have to know about them. The problem stems from using one simplification for many other meanings in which the traditional forms are totally different. Other examples include the folding of 隻 into 只; 鬆 into 松. The problem gets worse when three or more are folded into one character for the sake that they sound the same just for the purpose of stroke elimination. Just imagine if "two", "too" and "tool" are folded into "to"!

Well, 干(Gān)、幹(gàn) and 乾(Gān / qián) are three different characters in traditional Chinese system; however, they are all written as 干 in simplified Chinese system. 1. 干 equals to 乾 in some cases, such as 豆腐干; 干 also has other meanings such as in 天干 2. 幹 (gàn) can be used with other word to form a new word, such as 樹幹 The word can also mean: to do (幹什麼) something or to fxck (as a verb). It can also be used as a interjection, something like (fxxk, shit) 3. 乾 (Gān) means dry/ dried, etc. (乾燥) The word can also be pronounced as (qián), depending on the combination and its meaning, such as 乾隆、乾坤

It's a very complicated question. 干 in simplified Chinese mainly stems from three traditional Chinese characters which are 干, 幹 and 乾. 1. If 干 in simplified Chinese means ①shield, ②river bank, ③offend or ④to involve, its traditional form is 干(with pinyin gan in the 1st tone). 2.If 干 in simplified Chinese means ①trunk, ②main part of things or ③to do, its traditional form is 幹 (with pinyin gan in the falling tone). 3.If 干 in simplified Chinese means ①dry, ②empty or ③a kinship but not blood ties, its traditional form is 乾(with pinyin gan in the 1st tone). ps: in some time 乾 also has the meaning of "to do" and its pronunciation is gan in the falling tone. It is a special case that there is no simplified form of 乾 if it means something about the Eight Trigrams(in this case its pronunciation is qian in the 2nd tone ). There are some other traditional forms of 干. Such as 榦, 乹, 亁(this is not 乾...they are slightly different in character) ... In short, it is quite intricate...

喜欢用繁体的同学要注意了。

这乱简化汉字的后果。人雲亦雲！其实应该是“人云亦云”，一简化，云朵本应是雲朵，天上的雲就和说话的云成了一个，也就只好人雲亦雲了。