ALSO POSSIBLE: Half A Year LeftRemaining ALSO POSSIBLE: Half A Year Remaining

Today, it becamthere are one hundred eighty days left until the common exam for entering university. Today, there are one hundred eighty days left until the common exam for entering university.

You might also say "As of today, ..."

Today, it became oOne hundred eighty days leftremain until the common exam for entering universityI set for the common university entrance exam. One hundred eighty days remain until I set for the common university entrance exam.

I have many tasks to do. I have many tasks to do.

This is fine, but it is more common to say something like "I have a lot to do."

Since starting this summer vacation, I have spent three days studying, but I am concerned whether I will be able to prepare in time. Since starting this summer vacation, I have spent three days studying, but I am concerned whether I will be able to prepare in time.

might be better to say "I am concerned about whether" I think "concerned whether" is okay sometimes, but it feels unnatural here.

Since starting this summer vacation, I have spent three days studying, but I(I) am concerned whether I will be able to prepare in time. Since starting this summer vacation, I have spent three days studying, but (I) am concerned whether I will be able to prepare in time.

NOTE: The 2nd "I" should be deleted to make the sentence more succinct.

I must go to school during this vacation because I belong to a sports festival team and practicing it, andwe have to practice, and I have to take some make-up classes to relearn what I didn’t understand. I must go to school during this vacation because I belong to a sports festival team and we have to practice, and I have to take some make-up classes to relearn what I didn’t understand.

By "practicing it," do you mean that you have to practice for the sports festival? I made the correction that I thought made sense for that, but if you meant something else you might want to change the sentence in another way.

I must go to school during this vacation because I belong toam in a sports festival team and must practicing it, ande more, and also take some make-up classes to relearn what I didn’t understand. I must go to school during this vacation because I am in a sports festival team and must practice more, and also take some make-up classes to relearn what I didn’t understand.