How do you know its futur or present tense ?

When you say: yuki o furanai deshou. = it means "it probably won't snow"

But if I say:

"Sachiko wa manga o yomanai deshou. "

Does it means "Sachiko will probably not read a manga."
or
"Sachiko probably dont read manga."

?

Sachiko probably does not read manga.

Sachiko probably will not read manga.

Sachikoはたぶんマンガを読みません。

Sachikoはたぶんマンガを読まないでしょう。

雪は降らないでしょう。; Yuki wa furanai deshou.
サチコはマンガを読まないでしょう。; Sachiko wa manga o yomanai deshou.

These "deshou" is "guess" form. So you can use it in past tence.

昨日雪は降らなかったでしょう。; Kinou yuki wa furanakatta deshou.
I think/guess it didn't snow yesterday. = It probably didn't snow yesterday.

今夜雪は降らない/降りません。; Kon'ya yuki wa furanai/furimasen.
The speaker strongly believes "it will not snow tonight."


Similarly, "Sachiko wa manga o yomanai deshou" expresses the speaker's guess, and it can be used for both "in the particular case" and "custum."

サチコは(この)マンガを読まないでしょう。; Sachiko wa (kono) manga o yomanai deshou.
I think Sachiko will not read (this particular) manga. = Sachiko probably will not read (this particular) manga.

サチコはマンガを読まないでしょう。; Sachiko wa manga o yomanai deshou.
I think Sachiko does not have the custom to read manga. = Sachiko probably does not have the custom to read manga.

hope this helps

PERFECT explanations.

I understand now , thanks so much guys