What Happens When You Use hai and iie to Answer Questions
Let's say that you're beginning to learn Japanese, and that Mrs. Watanabe doesn't know any English at all. Let's see what would happen if you were to believe, wrongly, that you may say "hai" or "iie" whenever you'd say "yes" or "no" in English.
Kondono Nichiyoubi wa jikan ga arimasu ka ?
This "question" means "you are free next Sunday, right ?".
Suppose you have free time.
If you knew Japanese enough you would answer arimasu or hai. If, however, you don't know much Japanese, you might first think what would you answer in English ("yes"), and then remember that your teacher said "Japanese for yes is hai", and you'd say hai.
If so, you have been lucky. You've said what a Japanese would have said. Everybody will understand that you meant "you're right", hence that you'll have time. Mrs. Watanabe, who happens to speak with "masu" and "desu" to you all the time, will even think that you speak polite Japanese.
Now suppose you don't have free time.
An answer of arimasen "there isn't" would mainly be OK (a few excuses after it will make it sound even better, as we saw in the previous page).
However, suppose you just think "No." and then apply the rule-of-thumb "Japanese for no is Iie", so you say Iie. Well, Mrs. Watanabe will understand that you don't have any free time that Sunday. I'm not sure what will Mrs. Watanabe think about your politeness.
Anyway, just to be on the safe side, please use a longer answer such as Zannendesu ga arimasen or Hokano yotei ga arimasu. This cannot do you any harm, because that's what a polite Japanese would say.
Issho ni ikimasen ka ?
This means "won't you go with us?" The question Issho ni ikimasu ka ? would have exactly the same meaning.
Suppose you want to accept the invitation.
If you know Japanese enough, you'll answer any of the two questions with Ikimasu "I'll go" or Iidesune "OK" or Hai.
So, if you merely think in English "yes", and then apply the rule "Japanese for yes is Hai", Mr. Watanabe will know you'll go, and she won't find this answer strange.
Now suppose you want to refuse the invitation, by whatever reason (say, you are allergic to pollen).
If you knew Japanese well enough, an answer of Ikemasen "I can't go" would be fine. You might add some excuses if you wish to sound more polite, as for instance Sekkakudakedo ikemasen or the other phrases we saw in the previous page.
But suppose you don't think of that, and, instead, you think in English "no", then apply the rule-of-thumb "Japanese for no is Iie", so you say "Iie". Blunder!
The Japanese don't use Iie in this case because, well... because, as a Japanese would say in English, "this word is too strong". In fact, saying that it's "too strong" is an understatement. The word Iie in this situation does in fact mean something like "Of course not. What gave you that preposterous idea?", or "Of course not. I don't go with you absolutely".
So poor Mrs. Watanabe will know for sure that you won't go. But you won't hear from her again. Ever.
Kuruma ga arimasen ka ?
Be very careful here. This sentence does not mean "don't you have a car?". It means "you have no car, right?".
If somebody asked "you have no car, am I right?", how would you answer?
An answer of "you're right" or "you're right, I don't" would make clear that you don't have a car. That's exactly what an answer of Hai means: "you're right, I have no car".
On the other side, an answer of "you're wrong", taken literally, makes clear that you do have a car. But it doesn't sound very polite, even in English.
The Japanese word Iie, in this context, means the same thing, "you're wrong". Taken literally, it means that you do have a car. But it sounds even worse than its English counterpart, something like "You're plain wrong, you jerk. Of course I do have one".
Therefore, if you wrongly think that Kuruma ga arimasen ka means "don't you have a car?", Hai means "yes" and Iie means "no", you have 100% probability of getting understood backwards and 50% probability of unwillingly insulting somebody.
That's why my advice for foreign learners of Japanese is that they shouldn't use the words Hai and Iie at all to answer such negative questions. Please study carefully how do Japanese answer them.
Of course, this does not mean that the words Hai and Iie are not useful at all.
Uses of Iie
When somebody tells you anything like "you have a good car", "you're beautiful", "you speak good Japanese", "your house looks very big", or "your husband/wife is very smart", then you're expected to say immediately Iie, "not at all" while smiling nervously.
If a Japanese were to say Hai in such a moment, other Japanese would think he was boasting. Most Japanese don't like boasting at all.
Uses of Hai
The basic meaning of the word Hai is "I have understood what you just said". Sometimes it means "I agree with what you said" as well. There are other words with basically the same meaning, (like Ee or Unn) but Hai is the most polite. It never sounds impolite.
You may always use it
to say you agree,
to show you're hearing, and
to acknowledge a direct order from anyone with authority over you.
To accept invitations use Hai (or repeeat the verb); to refuse them, use excuses. At any rate, show you're grateful.
Answer conjectures with Hai (or repeat the verb) when you agree, and with excuses when you disagree. Don't use Iie AT ALL to answer conjectures unless you're very very sure you know what you're doing.
Akiramecha dame !
This means "Never give up".
What I've explained in these pages should not make you think that Japanese is horribly involved. It's not involved... it's just very different from English. That's why, when a Japanese learns English, he finds "yes" and "no" to be very difficult words which English speakers use in very surprising ways.