What exactly does -nasai do?
Minna-san,

I thought I wouldn't have any questions left for today, Yappari, I was wrong! :) More from the newbie-front!

Okay, so when I learnt 'yasumi' (vacation, time off) today, I immediately had to think of 'oyasuminasai' (good night), which, stripped down, begets us:

o-yasumi-nasai

So, assuming the 'o' functions like a built-in honorary prefix, as it were (or it is something else, but it's not relevant for my point), I got to wonder what exactly -nasai does. From what I can tell, often it feels like it functions as a kind of 'assuaged Imperative' (for lack of a better term); like:

Suwarinasai! - (Please) have a seat!

The 'suwarinasai' form does not appear in the verb conjugator:

http://www.japaneseverbconjugator.com/VerbDetails.asp?txtVerb=suwaru

Yet it exists; like:

Shukudai o shinasai! - (Please) do your homework!

And this 'softened' Imperative (surprisingly, almost) seems to work on 'oyasuminasai' too, so as to form something like: "(Please) take the off-time!" So, I pushed my luck, and tried it on 'okaerinasai' (in response to tadaima!) too:

o-kaeri-nasai

And, indeed, kaeri appears to be "return, coming back." So far so good. But the Imperative sense, however softened, seems lost here (or manifests itself in a way invisible to me). So, my simple question is, what does -nasai do precisely? My gut tells me I can't be too far off, but I need just this small nudge, it seems, to get me fully on-track. Either that, or I derailed myself completely. :)

Arigato gozaimasu!Wozitoya-san, thank you for explaing the verb conjugation rules.

Okay, minna-san, let me see if I can fit this into a single rule my brain can grasp, and which will allow for irregular verb as well. What if I break of the -masu form of each verb, and replace it with -nasai, would that work?

kuru - ki-masu - ki-nasai
suru - shi-masu - shi-nasai
wakaru - wakari-masu - wakari-nasai
iu - ii-masu - ii-nasai
yomu - yomi-masu - yomi-nasai
miru - mi-masu - mi-nasai

Seems to work. Would this be a valid way to go about it?

Thanks!Well, seems it also works on taberu (not having '-imasu'):

taberu - tabe-masu - tabe-nasai

So, I'm just assuming I'll be pretty safe with the '-masu' rule.

Question

What exactly does -nasai do?
Minna-san,

I thought I wouldn't have any questions left for today, Yappari, I was wrong! :) More from the newbie-front!

Okay, so when I learnt 'yasumi' (vacation, time off) today, I immediately had to think of 'oyasuminasai' (good night), which, stripped down, begets us:

o-yasumi-nasai

So, assuming the 'o' functions like a built-in honorary prefix, as it were (or it is something else, but it's not relevant for my point), I got to wonder what exactly -nasai does. From what I can tell, often it feels like it functions as a kind of 'assuaged Imperative' (for lack of a better term); like:

Suwarinasai! -> (Please) have a seat!

The 'suwarinasai' form does not appear in the verb conjugator:

http://www.japaneseverbconjugator.com/VerbDetails.asp?txtVerb=suwaru

Yet it exists; like:

Shukudai o shinasai! -> (Please) do your homework!

And this 'softened' Imperative (surprisingly, almost) seems to work on 'oyasuminasai' too, so as to form something like: "(Please) take the off-time!" So, I pushed my luck, and tried it on 'okaerinasai' (in response to tadaima!) too:

o-kaeri-nasai

And, indeed, kaeri appears to be "return, coming back." So far so good. But the Imperative sense, however softened, seems lost here (or manifests itself in a way invisible to me). So, my simple question is, what does -nasai do precisely? My gut tells me I can't be too far off, but I need just this small nudge, it seems, to get me fully on-track. Either that, or I derailed myself completely. :)

Arigato gozaimasu!Wozitoya-san, thank you for explaing the verb conjugation rules.

Okay, minna-san, let me see if I can fit this into a single rule my brain can grasp, and which will allow for irregular verb as well. What if I break of the -masu form of each verb, and replace it with -nasai, would that work?

kuru -> ki-masu -> ki-nasai
suru -> shi-masu -> shi-nasai
wakaru -> wakari-masu -> wakari-nasai
iu -> ii-masu -> ii-nasai
yomu -> yomi-masu -> yomi-nasai
miru -> mi-masu -> mi-nasai

Seems to work. Would this be a valid way to go about it?

Thanks!Well, seems it also works on taberu (not having '-imasu'):

taberu -> tabe-masu -> tabe-nasai

So, I'm just assuming I'll be pretty safe with the '-masu' rule.

Answer

Just only a small thing that you are in doubt "nasai", it is equivalent to the English word " please". By definition, it works with the verbs +nasai, 

all words in Japanese have the stem, you need to chance it to rule (2)

"kaeri" it is wrong by itself, it's stem is "ru". So, the correct word by itself is kaeru. In conjugation, the rule for "ru" is (1)ら, (2)り, (3)る, (4)る, (5)れ, (6)れ. In rule 2, it is change the stem from  るto り.

oyasuminasai, yasumi is wrong by itself, it's stem is む. yasumu (休む). The conjugation rule for む is (1)ま, (2)み, (3) む, (4) む, (5)め, (6)め. 

"O" is not related to the verb yasumu, just for being more polite.

Answer

We often say " o kaeri nasai ",and " o yasumi nasai ".
おかえりなさい。　おやすみなさい。
This " o " means, honorific expression.
ex) o namae ,  


When we say to friend or family, we say " o kaeri" ," o yasumi".
To the peple who is unfriendly, is father and mother,
we say " o kaeri nasai ",and " o yasumi nasai ".
In this case, nasai is polite language.


"Syukudai wo sinasai" .This wo doesn't mean honorific expression. 
宿題　を　しなさい。

Answer

This is a very old post, but I thought I would leave this here.  Sometimes, you can't directly translate "this means this" and get the best result.  Sometimes you have to find the general feeling of something and then describe that feeling in the other language.  In the case of -nasai, I get the feeling of the suffix as not a pleading (please) so much as a hopefulness.  So depending on the situation, you may write that in English as please, or possibly I hope you.  Try it with words listed...  Oyasumi-nasai could be written in English as "Please have a good night" or "I hope you have a good night" and either makes sense.  With the one that doesn't seem to work, o-kaeri-nasai, try "I hope you return."  It's also kind of implied that you hope they will return safely.

Disclaimer:  I am very new to learning Japanese, yoroshiku onegaishimasu.

Answer

nasai is Japanese " please"