You're English is pretty good. "I thought my mother said that she believed it will snow tomorrow" is a perfectly valid sentence and you will find similar spoken in English.
I have very little experience with Japanese, so forgive me, but I'm going to give an attempt at your sentence.
Let's start with the easy part...
It will snow tomorrow:
Ashita ni kousetsu suru.In your first sentence, the は particle for 明日 makes me think that tomorrow is the one that thinks, when you are the one thinking. For that reason, I'd associate が with 明日 instead, as tomorrow isn't the topic of discussion, but rather what you think.
I don't think 雪になる is correct. になる I believe is used to join NA-adjectives to
suru, not nouns. If you want "to do snow," I think
kousetsu suru is the only option available.
EDIT:First, I'm an idiot for confusing
naru with
suru. Though I don't think "become snow" makes much sense.
Second, I found the following while researching something else--
Yuki ga takusan furimashita./"A lot of snow fell from the sky."--used for "It snowed a lot."
Furu would be the verb of choice for your original sentence rather than
naru. (I'd keep
suru with
kousetsu, however). /EDIT
My mother believes:
Haha wa shinjiru/shinzuruI'm fairly certain between
shinjiru and
shinzuru, one of these is transitive and the other is intransitive, I just have no idea which is which, and in this case I honestly don't have much of an idea as to what would be appropriate in this case (I think intransitive, as she isn't believing in something). I'm using は here, but in the full sentence, as the mother still isn't the topic of discussion, it'll be が.
My mother believed it would snow tomorrow:
Haha wa ashita ni kousetsu suru no ga shinjitaHere we nominalize the entire
ashita ni kousetsu suru clause with the addition of
no.
My mother said she believed it would snow tomorrow:
Haha wa ashita ni kousetsu suru no ga shinjita to itta.と is used to mark the end of a quotation, so we can pair it with
iu and come up with "My mother said 'I believed it would snow tomorrow.'" That sounds a bit strange to me, so I would go with
Haha wa ashita ni kousetsu suru no ga shinjiru to itta.
I thought my mother said she believed it would snow tomorrow:
Haha ga ashita ni kousetsu suru no ga shinjiru to itta watashi wa omotta.Mother isn't the topic of the sentence here, so she gets a が. Instead, I am the subject, so we move the は. The structure basically says "All that stuff up front, that's what I thought." I think it might be possible to replace the
watashi wa with a
no wo which would work similar to the
no ga. Then, if you want the whole statement to be polite, you use
omoimashita at the end instead.
And that's my attempt. Thank you for putting up with me!
