expressing 'I might do something' in Japanese

I'm not japanese, nor am I an expert, but I can say what I believe.

kamoshiremasen/ kamoshirenai means that somethibg pretty much has a 50% chance of being like you say. You're basically making a guess. It might be like that, and it might not. However when making negative sentences its often used as a soft refusal. So something like " ashita mise ni ikanai kamoshiremasen" would often be read as an absolute as opposed to actually having a 50% chance of happening, so be careful with it.

deshou can only be used in reference to other peoples action and not your own. Kamoshiremasen can be used for both.

So, correct way to say for example "i might go to the library tomorrow" would be " ashita (watashi wa) toshokan ni iku kamoshiremasen

Wrong: ashita watashi wa toshokan ni ikudeshou.

Correct use of deshou: Ashita Ane wa toshokan ni iku deshou = My older sister will probably go to the library tomorrow.

Note that deshou implies you are more certain than kamoshiremasen. Deshou = probably, rather than might. If kamoshiremasen is 40-60% chance of happening then deshou is more like 80-99% if that makes sense?

Sorry if this is unclear. English is not my first language and neither is japanese. :3

Hi, and thank you for the reply! No this is very clear =) oSo - if i wanted to say:
' i might study history later this afternoon' with the idea that it is 50-50 chance of happening, would this be right:

' Watashi-wa kyo no gogo ni, ato de rekishi suro koto ga kamoshiremasen.'

And if I give a sentence in the negative, using kamoshiremasen, does it convey that I definitely will NOT do the action?
For example, 'Tonight I will not drink alcohol'
Would this be ' Kon'ya watashi-wa (alcohol, lol don't know how to say in Japanese?) nomeru kamoshiremasen.

However what is the difference between saying this and 'Kon'ya watashi wa alcohol-o nomimasen? Isn't it the same idea expressed in two different ways with regards to the sentences?

1)
~ kamo shirenai basically means "maybe". It literally translates to "I can't even know whether or not ~"
The example sentence you gave is correct.

2)
You are correct that you would negate the verb to negate the idea, but "iku nai" is not the correct way to do this. It's "ikanai"

additionally, dewa (or ja) can only follow a noun, and doesn't signify negation by itself (though it can imply negation).
you could say "watashi wa mise ni iku no de wa nai", but the "~no de wa nai"/"~n'ja nai" phrasing implies a strong certainty, so "watashi wa mise ni iku n'ja nai kamo shiremasen" does not make any sense.

3)
Tabun is "probably". Deshou is... a little more difficult to get a handle on, because of its versatility, but "watashi wa mise ni iku deshou" would be contextually recognized as roughly equivalent to "watashi wa tabun mise ni ikimasu", like "I think I'll go to the store" and "I'll probably go to the store". Putting both "tabun" and "deshou" in the same sentence deintensifies it, like "I think I'll probably go to the store" -- less certainty than either form alone, but still more certainty than ka mo shirenai.

to summarize: the certainty level of "ka mo shirenai" is basically "I have no idea" whereas deshou and tabun both imply likelihood of the stated case.

1- Regarding 'ikunai' is this used whenever I say I will not go somewhere in the present or future?

So for example, ' I will not go to my friend's house' would it be 'Watashi-wa watashi no yūjin (an aside - I note this word is also used in place of tomodachi to refer to friend, do they depend on formality level or context?) no ie ni ikanai.'

2 - Would you mind giving an example or two of using the phrase 'dewa/ ja arimasen' and I didn't quite follow, this can only follow a noun?

Like if I wanted to say 'The dog is not small' Can't I say: Inu wa chīsai de wa nai or de wa arimasen, though this follows adjective chisai?

1) There's two ways to express a negative of a future verb:
the plain (nai) form, and the formal (masen) form.
there's a simple pattern to get the nai form from the plain (dictionary) form of verbs, though there's two types of verbs.

for ichidan (one-column) verbs (most verbs that end in "*eru" or "*iru"), simply replace the "ru" with "nai"
ex: taberu -> tabenai
I won't eat that sort of thing -> sonna mono wo tabenai

for godan (five-column) verbs (all verbs that end in something other than "ru", some that end in "ru")
replace the last kana (*u) with the kana from the "a" column (*a), (exception: う becomes わwhen conjugating into a "*a" suffix), then add nai
ex: nomu -> nomanai, iku -> ikanai, kau -> kawanai.
exception: aru (to be) -> nai, not aranai.

I won't go to the library today -> kyou wa toshokan e ikanai.

For the formal version, take the masu form, and just replace the "su" with "sen".

If you're not sure how to get the masu form of a verb, it's pretty simple
ichidan verbs: ru -> masu ex: taberu -> tabemasu
godan verbs: *u -> *imasu ex: nomu -> nomimasu, iku -> ikimasu, kau -> kaimasu

2. "dewa/ja arimasen/nai" is essentially, but not quite exactly the negative of "desu".
ja is actually just a phonetic contraction of dewa. it means exactly the same thing, and doesn't even affect formality level. In fact, saying "dewa" instead of "ja" can sound a bit stilted.
Nai is the plain (that is, informal) form of arimasen.
I am not a student -> "watashi wa seito ja arimasen"
You aren't my friend -> "anata wa watashi no tomodachi ja nai"

As for your question, that exact mistake is something I made in my first semester of studying Japanese, probably for exactly the same reason you would: the place you'll trip up if you think of "ja arimasen/nai" as the negative of "desu/da" is with adjectives. The fact is that there isn't a direct negative of desu/da, and "ja arimasen" is the negative of "de arimasu" which means the same thing as "desu" but has some important syntactic distinctions.
the "de" in "de arimasu" is used to let the noun modify the verb. for an adjective to modify a verb, you have to conjugate it to an adverb, which follows a simple pattern.

i-> ku

So to say your example "the dog isn't small" properly, you get "inu wa chiisaku arimasen".
which is the negative of "inu wa chiisaku arimasu", which is a really really stilted way of saying "inu wa chiisai desu".

Now, you may occasionally hear people say "ja nai" or "ja nakute" or "ja arimasen" after adjectives, but when this occurs it's actually a sentence fragment negating everything they'd just said, grammatically this is the same thing as the American English "Not." but the implication is "on second thought".

For instance, you might hear "kono inu wa chiisai. ... ja nai" , which is "This dog is small. Wait, no it isn't."

「行くかも知れない」(ikukamo sirenai)＝"it's not possible to know I go or not" so, I might go (but nuance is maybe you expect I won't go, but I let you know now maybe I will)
「行くでしょう」(ikudeshou)＝"very probably, (I) will go" "odds are as anyone sees it, (I) will go" as others put just putting でしょう (deshou) at the end of declarative sentence is more common for talking about other people or observations outside yourself, but technically it works
「行かないかも知れない」(ikanaikamosirenai)="it's not possible to know if I won't go, or not" so, I might go (but nuance is you expect I will go, but I let you know now maybe there's a possibility I won't)
「たぶん行くと思います」(tabun iku to omoimasu)＝"I think probably I will go" (if you use たぶん(tabun) maybe it's the most common way to talk about yourself?) the 「…と思います」(...to omoimasu) is important it makes it very different from でしょう (deshou) or similar because it shows your initiative

「たぶん行くでしょうか」(tabun iku deshouka) sounds odd to me (I can't say why) but 「たぶん行くでしょう」(tabun ikudeshou) sounds better and that would be about other people. I guess because たぶん (tabun) just means "odds are that" and so it's not about a question at all.

I can't do grammar... XD

ETA: oops, put transliteration in

hahah of course you can clearly expound on the complexities of Japanese grammar, my goodness there is so much. It's incredibly vast and I don't know how I'll ever manage to grasp it all. ;_;

Thank you for these exsamples! I will need to re-read these to really make sure I follow...