I taught ASL for three years here in Tokyo, and I’ve seen some interesting errors pop up among Japanese students learning ASL, that would probably never come up among hearing students in the US. But when you think about them, they make perfect sense.
A common error is mouthing a Japanese word while signing an ASL sign. For example, mouthing ‘aru’ while signing out ‘have.’ “Aru” is indeed the closest word to “have,” but mouthing it would possibly render the whole sign or even the whole message incomprehensible to a native signer of ASL. Seeing that reinforced to me the dictum that a sign is not limited to the hands, but the whole body. It should be the body signing, not the hands signing. The subtle movement of the head, the mouth shape, the shoulders’s tilt. Even Deaf people, native signers, often have the misconception that the sign is confined to the hands, and it showed in my students, some of them Deaf themselves.
Mouthing Japanese words betrays that they are thinking in Japanese, not ASL. Even if you mouth it in English, it is no better – you should not be thinking in English anyway, although many of us still do.
Another one is the handshape. In JSL there is a handshape that does not exist in ASL: the open-hand, ring finger out. The closest handshape in ASL is “seven,” but that uses the thumb. In Japan, that sing is used for the word “medicine” (kusuri). I teach them the ASL sign for “medicine” (using the middle finger) and they invariably use the wrong finger – I do not blame them, the signs are so similar. However, they sometimes use the ring finger instead of the middle finger in another signs, like “SICK-OF.”
Word order is particularly interesting, because in Japanese the basic sentence structure is S-O-V, and ASL often follows that structure as well, expect you usually repeat the S, like “YOU-HOMEWORK-FINISH-YOU?” A student with a meticulous bent would sign out “I-STORE-GO,” which isn’t so bad, expect that he mouthed it out in perfect Japanese: “WATASHI-OMISE-ITTA” which gave me a linguistic aneurysm. I’ve never heard a word in my life, but it must be like listening to two different languages at the same time, while understanding both. He’s halfway there, yet strangely so far away.
I suppose what I want to say to prospective students: be aware of your linguistic preconceptions and tendencies, what language you are thinking of, and try to break out from it. But that is difficult for those with weak CL or gesture skills. And the more signing models you learn from, the better.