Teacher, your announcement was too delayed... Teacher, your announcement was too delayed...

(1) "Announce" is a verb. "Announcement" is the noun. (2) "Delay" is a verb. "Delayed" is the adjective.

Teacher, your announcement was too delaylate... Teacher, your announcement was too late...

or "too delayed" but that sounds a bit formal

Today, I felt very coldnervous because of an accident... Today, I felt very nervous because of an accident...

This is fine, but based on your Japanese version, I might say "Today something very nerve-racking happened."

Today, I felt very coldanxious because of an acincident... Today, I felt very anxious because of an incident...

(1) "felt very cold": 寒く感じた. This is not the same as "felt very anxious". (2) "Accident" and "incident" are very similar words, but they have different nuances. "Accident" refers to an event that happened unintentionally and typically damages something or injures someone. On the other hand, "incident" refers to any significant event. All "accidents" are "incidents", but not all "incidents" may be called "accidents". In this case, "incident" is the more natural-sounding word.

In the first place, my teacher told us that today's class was held by myself at my home. In the first place, my teacher told us that today's class was held by myself at my home.

I am not exactly sure how to correct this. Part of the issue feels like the phrasing. You say "today's class was held by myself at my home" This partly makes me have a vegue feel that the class was still happening. Perhaps over some video chat. If you where trying to say that class material was given to go over at home then you might say it differently. "told us that today's class was self-study with material she gave." I might say that. I think part of the issue is the "teacher told us." It seems weird to have the statement after of "today's class was held by myself at my home." I wonder why the teacher would include the at home statement. Anyway, I am trying to circle what is confusing or off to me. But let me include ways I might say something like that. They may not be exactly what you meant but hopefully they help you. 1. "In the first place, my teacher told us that today's class was self-study." (so maybe the teacher said, "Students, I am sick, feel free to go over the material at home." 2. "In the first place, my teacher told us that today's class was canceled, and that we could study at home." (this just expands so the home part is included. 3. "In the first place, my teacher told us that today's class was canceled." (This would be what I would expect a teacher to do, though I guess some might say read, "read this," or "watch that."

In the first placeitially, my teacher told us that today's class was held by myself at myould be self-study at home. Initially, my teacher told us that today's class would be self-study at home.

(1) The nuance of "in the first place" is actually really difficult to explain. I might have to try explaining it in a separate comment when I find the time... In any case, it sounds very unnatural to me here. (2) 自習: self-study.

In the first placeOriginally, my teacher [had] told us that for today's class was held by myself at mye'd be doing self-study at home. Originally, my teacher [had] told us that for today's class we'd be doing self-study at home.

In your sentence, it sounds like the teacher and other students were going to come to your house for class

Usual class starts from 9 a.m., so I started to study from that time, and I continued it to around half 10 o'clock10:30. Usual class starts from 9 a.m., so I started to study from that time, and I continued to around 10:30.

I feel like you could say the "continued it," but it feels more natural to me to not include the "it." In my head, I know you are studying so continued makes sense and the it seems not needed or extemporaneous. I wouldn't say half 10 o'clock. You could say half past 10 o'clock. You can't say half before 10 o'clock. In my mind. So, I would just say 10:30. Though, I do get the meaning I think. Unless you meant 9:30.

Usual class starts from 9 a.m., so I started to study from that time, and I continued it to around half 10 o'clockdoing so until around 10:30 a.m. Usual class starts from 9 a.m., so I started to study from that time, and I continued doing so until around 10:30 a.m.

Since you used numerals for "9 a.m.", I would suggest using numerals for "10:30 a.m." as well. However, for your information, "10:30" can also be written as "ten-thirty" or "half past ten".

Usual class starts fromat 9 a.m., so I started to study fromat that time, and I continued it totill around half [an hour] past 10 o'clock. Usual class starts at 9 a.m., so I started to study at that time and continued till around half [an hour] past 10 o'clock.

or just "until 10:30" "o'clock" in writing sounds a bit quaint

After that, I checked my Teams (an app's name)'s announcement(s) and I(I) was very surprised. After that, I checked my Teams (an app's name) announcement(s) and (I) was very surprised.

(1) "Teams' announcement(s)" sounds rather unnatural. When referring to an app, we will drop the apostrophe. (2) Instead of "announcement", you can also say "notification". This might be the more common word to use. For example: "Line notification", "Instagram notification", etc. (3) If you want, you can omit the second "I", because you've already defined the subject of the sentence.

After that, I checked my Teams (app's name)'s announcementthe app) notifications and I was very surprised. After that, I checked my Teams (the app) notifications and was very surprised.

Or, closer to your Japanese: "After that, I got a notification from Teams (the app) so I checked it and was very surprised."

It said that "Today, we'll have a(a) class online"! It said that "Today, we'll have (a) class online"!

Omitting the "a" might sound more natural. This is because you're specifically referring to "today's class". "A" doesn't have the same level of specificity.

It said that "Today, we'll have a class online"! It said that "Today, we'll have class online"!

The email, to my surprised, arrived at the time which will be 9 o'clock in 2minutes2 minutes before 9 o'clock. The email, to my surprise, arrived 2 minutes before 9 o'clock.

"To (someone's) surprise" is a set expression.

The email, to my surprised, arrived at the time which will be 9 o'clock in 2minuteshad been delivered 2 minutes before 9 o'clock. The email, to my surprise, had been delivered 2 minutes before 9 o'clock.

I can't believe that itsuch a thing really occurred... I can't believe that such a thing really occurred...

"It" might be a little vague. "Such a thing" can clarify what "it" is referring to.

I still can't believe that ithis really occurrhappened... I still can't believe that this really happened...

"occur" is a bit formal/academic "still" or "even now" = 今でも