I have been taught that the sentence 我刚到了 (wǒ gāng dào le) is not correct because 了 cannot be used as a complement of 到. Is this right? If so, why?
我刚到了 is perfectly understandable in oral communication. Your teacher probably thinks this is verbose because:
刚 = just
了 = already
我刚到了 = I just arrived already
Before you provide more context, I would say this could be perfectly understood by people except that it might not sound that natural.
Depending on the context, you would prefer to say: