I forgot to close the window
Ami janala bondho korte bhule gechhi.
The door was open
Dorja khola chhilo.
I forgot to get my change
Ami taka [bhangti korte] / [bhangate] bhule gechhi
(I'm assuming you mean "change" as in loose change in coins.)
I left my bag here have you seen it?
Ami amar bag ekhane fele gechhilam; dekhechhen?
I forgot who you are
Ami bhule gechhi apni ke.
I forgot all about that
Ami otar kotha puropuri bhule gechhi.
Sometimes i forget things
Majhe majhe ami kichhu jinish bhule jai.
It is cloudy today
Ajke megh korechhe.
Ajke akash meghla.
It looks like it will rain
Mone hoy brishti hobe.
Will there be many floods today?
Ajke ki bonna hobe?
Some roads are flooded it is dangerous.
Kichhu rasta panir niche dube gechhe; eta bipodjonok.
You could get electricuted if a wire is on the ground.
Jodi kono tar rastae pore thake, apni [toritahoto hote] / [biddutsprishto hote] / [shock khete] paren.
("Shock" is, of course, a word borrowed from English. Relative to the other two options, this is the term that would most commonly be used in colloquial Bengali.)
I am reading the news.
Ami khobor porchhi.
I am finished reading the news now.
Amar khobor pora shesh.