Questions about example sentences with, and the definition and usage of "Rainny"

A: × I'm gloomy in rainny day
✓ I get gloomy on rainy days.

I feel gloomy on rainy days.

A: × It's rainny day :( but it's okay bc I heard the good news.
✓ It's rainy day :( but it's okay bc I heard the good news

× Which is about my new department.
✓ about my new department.

× We change our department every new year and I might have an opportunity that I could go good department even though it's not sure.
✓ We change our department every new year and I might have the opportunity to go to a good department, but it's not certain.

× For me, 'Good department' means I don't have a lot to do so I get less stress.
✓ For me, a 'Good department' means I don't have a lot to do, so I get less stressed.

× If it happens, I will study English more and focus on me like doing exercise, listening a voice of my mind and so on.
✓ If it happens, I will study English more and focus on me like exercising, listening to myself and so on.

× My another big plan is living alone!!
✓ My other big plan is to live alone!!

× I've never used to live alone.
✓ I've never lived alone before.

× Also I'm kinda afraid of many thing though.
✓ Also I'm kinda afraid of many things though.

× Anyways, I should go back to work I just had coffe break.
✓ Anyway, I should go back to work. I just had coffee break.

Your English is very good! :)

A: it's raining. I'm worry about my things.

it's depends what are you talking about

A: "It's a rainy day... ☔️
But, have a nice day!!"

Good job :)

A: "The" is used to mark the word weather as a direct object in the sentence.

It's like talking about the newspaper. You wouldn't normally ask "What is news?" you would ask "What is the news?". When you read it like that, "What is weather?" or "What is the weather?" it might be easier to see.

In the sentence you have, you actually can leave out "the" and it would still be correct, but it is almost always used as "the weather".