foryoudaoicibaDictYouDict[for 词源字典]
for: [OE] For comes from a prehistoric Germanic *fora, which denoted ‘before’ – both ‘before’ in time and ‘in front’ in place. For itself meant ‘before’ in the Old English period, and the same notion is preserved in related forms such as first, fore, foremost, former, from, and of course before. Germanic *fora itself goes back to Indo- European *pr, source also of Latin prae ‘before’, pro ‘for’, and primus ‘first’ (whence English premier, primary, etc), Greek pará ‘by, past’, pró ‘before’, and protos ‘first’ (whence English protocol, prototype, etc). and English forth and further.
=> before, first, fore, former, forth, from, further, premier, primary[for etymology, for origin, 英语词源]
for (prep.)youdaoicibaDictYouDict
Old English for "before, in the sight of, in the presence of; as far as; during, before; on account of, for the sake of; in place of, instead of," from Proto-Germanic *fur "before; in" (cognates: Old Saxon furi "before," Old Frisian for, Middle Dutch vore, Dutch voor "for, before;" German für "for;" Danish for "for," før "before;" Gothic faur "for," faura "before"), from PIE *pr- (see fore (adv.)).

From late Old English as "in favor of." For and fore differentiated gradually in Middle English. For alone as a conjunction, "because, since, for the reason that; in order that" is from late Old English, probably a shortening of common Old English phrases such as for þon þy "therefore," literally "for the (reason) that."