Goshinsetsu ni, tasukarimashita.
You’ve been a great help. It was very kind of you.
If, as most speakers would probably agree, the most frequently used expression of gratitude in contemporary Japanese is arigatou goza-imasu (“Thank you”), then these are perhaps the next most commonly heard words of thanks. (Actually, as in the example below, the two expressions are frequently used in combination.) More so than arigatou gozaimasu, however, this phrase conveys a feeling of indebtedness. When another person has done you a favor at your own request or has simply provided a helping hand out of the goodness of her heart, this is the phrase to use. Equally useful are such variants as:
Goshinsetsu ni arigatou gozaimashita. Okagesama de tasukarimashita.
Thank you very much for your kindness. All’s well now, thanks to you.
It should be stressed that these words convey a sense of gratitude more emphatic than you’d ordinarily show, say, to the waitress who brings you your coffee. Rather, this is the sort of thing you might say to a stranger who takes the trouble to give you explicit directions when you’re lost, or who hurries after you to return the wallet you dropped unknowingly, or who volunteers to help you carry something heavy, or who rescues you from a drunken boor on the train. Naturally, the kindness of others is never so keenly appreciated as when it is needed most, and at such times it is important to make an appropriately elaborate—and sincere—demonstration of thanks.
Occasions may arise when some kind soul helps you out of a really tight spot and words simply aren’t sufficient to communicate the depth of your gratitude. Imagine that a complete stranger has found your missing handbag and brought it to your home, with all the contents intact. In such a case, words of thanks delivered on the spot just aren’t enough, so you might ask how to reach the person at his home:
Douka, onamae to gojuusho o okikase kudasai.
Would you be so kind as to give me your name and your address?
You can then make a proper show of thanks by calling on your benefactor the very next day, usually bearing a small (and often edible) token of your gratitude.
The same course of action would be called for if, for example, you lost your wallet and a passerby lent you the train fare home. In this case, the kindly stranger might well decline the customary elaborate show of gratitude, saying something like:
Iya, nanoru hodo no mono de wa arimasen.
Look, it’s no big thing (not important enough to take credit for).
At this point, the only gracious thing to do is to accept the favor gratefully and, with a solemn bow, sincerely state your thanks:
Goshinsetsu wa wasuremasen.
I won’t forget your kindness.
When the person lending you a hand or doing you a favor is an acquaintance, rather than a stranger or passerby, it’s considered appropriate to express your thanks with the words:
Itsumo osewa ni narimasu.
I’m forever in your debt.
This expression is, however, perilously similar to the ubiquitous phrase Itsumo osewa ni natte orimasu, the formulaic phrase with which neighbors politely thank one another for being neighbors or business people greet their patrons and suppliers over the telephone, so a good deal of care should be taken to make it sound truly grateful rather than perfunctory.
– Source: A handbook of common Japanese phrases –