Question: Informal form of desu.

[origamiboxers]

Gahh... Exchange students are coming tomarrow and I forgot how to use 'desu' informally. All my japanese ability dies when the exchange students arrive! xp  

[HotaruTomoeIAA]

You mean you just need the verb form charts? *is still a from-the-text student at the moment, but can conjure that up*

desu:

Plain Present: da
Negative Plain Present: dewa nai (or ja nai)

Plain Past: datta
Negative Plain Past: dewa nakatta (or ja nakatta)

Plain Probable: darou
Negative Plain Probable: dewa nai darou (or ja nai darou)  

[chinotenshi]

Bah. Don't worry about knowing the informal of "desu" or not. They'll be quite impressed just if you can use the formal correctly as many Japanese cannot use much of the formal vocabulary anymore...

But if you really want to know,

desu -> da
dewa arimasen -> ja nai
deshita -> datta
dewa arimasen deshita -> ja nakatta

And really, you only need that after nouns and -na adjectives (i.e. "daijoubu da." for " I'm okay"). -i adjectives can be said informally just by not saying "desu" at all (i.e. "takai" for "It's expensive!").  

[origamiboxers]

ah. okay, thanks guys.  

[origamiboxers]

Wahh. My exchange student was lame. She totally ignored me.

Whats plain probable?  

[Nagaikami]

origamiboxers

Wahh. My exchange student was lame. She totally ignored me.

Whats plain probable?

It happens. Have a thing for Japanese females (especially the schoolgirl variety)? Best way to solve that is to actually go to Japan. Believe you me, meeting the real thing will set you straight. Asian Fever is a disease, but I believe in a cure.

For here who is a Japanese female, or dating one, I mean no real offense. I just mean to say that fanboy fantasies are easily and deservingly crushed. For more on this topic, read the sage words of Asrael.

As for gammar stuff, the plain form of "deshou" is "darou."  

[chinotenshi]

Plain probable is usually translated as "I think" if you were wondering what "plain probable" means ^^;  

[HotaruTomoeIAA]

Probable as in something that will probably happen.

Japanese Grammer by Akiyama & Akiyama

THE PRESUMPTIVE MOOD

Sometimes called the "probable" mood, this has no exact equivalent in English, although it is important in Japanese. It enables speakers to use a verb form that can express probability, belief, or intention without being too direct. This mood has two sets of forms, one indicating probability, the second indicating intention.

darou, deshou form

This form expresses the speaker's belief that something will probably happen. It is easily constructed by adding darou or deshou (plain and polite, respectively) to the pain, or dictionary form, of the verb. This works for the negative of the plain form, as well, and also for the past forms.

Examples the book gives are:

Jon san wa, tsukareta deshou. I suppose John got tired.
Ano hito wa, Nihon ni iku deshou. He'll probably go to Japan.
Otenki ga, warui deshou. The weather will probably be bad.

I hope that answers your question about probable. sweatdrop

As for the exchange student, are you sure she wasn't just shy? Or uncomfortable? Sometimes a little attention is too much, you know. Give her some time, maybe?