I received a very bad news yesterday.

The news was that my brother will also start online classes because there are a lot of people who have caught covid-19.

Because there are a lot of people who catch the covid-19.

This sentence was a sentence fragment so I deleted it and appended it to the previous sentence.

Yes, it is a good news for my brother, but it is a nightmare for me because I have had to teach him how to do his homework every day since then.

Because I have to teach him to do his homework every day since then.

This was another sentence fragment.

I'll go bananas when I teach him math.

The news was that my brother will also start online classes., because there are a lot of people who are catching Covid-19

I combined the following sentence for better flow

Because there are a lot of people who catch the covid-19.

Yes, it is a good news for my brother, but it is a nightmare for me.

Because I have to teachaught him to do his homework every day since then.

What tense are you using here? I put this tense because it sounds like you started in the past and still doing it now.

I'll go bananas when I have to teach him math.

I received asome very bad news yesterday.

When you receive news, you do not receive "a" news. Instead, use "some" news. or simply: I received (very) bad news.

Instead of using news altogether, you can say "I came to know something really bad yesterday" or "I found out something really bad yesterday"

The news was that my brother will also start online classes.

you can also say "... will also be starting online classes."

Because there are a lot of people who are catch theing covid-19.

you can simply say covid or corona.

Example sentence: People are catching [the flu/ influenza, the common cold, covid] in large numbers at the moment.

Yes, it is a good news for my brother, but it is a nightmare for me.

Try to add a sentence on why this is good news for your brother. (Does he not like going to school? What year/ grade is he in at school?)

Because I have to teach him to do his homework every day since then.

(It's bad news for me,) because I will have to help him with his homework once/ when his classes go online.

I'll go bananas whilen I teach him math.

I'll go bananas having to teach him maths - is also fine.

To go bananas, as you know, means to go crazy. Another way of saying something will drive you crazy, is to use, "it'll drive me mad."

Remember that it's math in American English and maths in British English. Use whichever is appropriate for your exam requirements/ learning goals.

I received asome very bad news yesterday.

“News” is uncountable.

Yes, it is a good news for my brother, but it is a nightmare for me.

Insert a blank space after the comma.

BThis is because I will have to teach him to do his homework every day sincefrom then on.

中文：昨天晚上我收到了一个坏消息，消息是我的弟弟也要开始线上上课了，因为有太多人感染了新冠病毒，这是非常危险的，对于我来说这是一个噩梦，对我弟弟来说还是一个好消息，因为我必须从今以后每天都要教他做作业，每次我教他做数学的时候我都会疯掉。

I received asome very bad news yesterday.

BThis is because there are a lot of people who catch the cught Covid-19. OR The news was that my brother will also start online classes because there are a lot of people who caught Covid-19.

It is very dangerous outside.

What is very dangerous? I am assuming the state of the outside environment here.

Yes,itIndeed, this is a good news for my brother, but it is a nightmare for me.

BThis is because I have to teach him to do his homework every day since thefrom now on.

Again, you can connect this sentence to the one before like in the other edit I made.

I'll go bananas when I/crazy/insane when/if I help teach him math.