As you probably know, coronavirus ihas spreading all over the world. As you probably know, coronavirus has spread all over the world.

Your other sentence is fine, but I'd use the past here to sort of emphasize it's already everywhere as opposed to "slowly" spreading

As you probably know, the coronavirus is spreading all over the world. As you probably know, the coronavirus is spreading all over the world.

I think "is spreading" is fine. It isn't yet in every country, and some countries have just 1 case, but "has spread" isn't wrong.

As you probably know, the coronavirus is spreading all over the world. As you probably know, the coronavirus is spreading all over the world.

There are some people who buy so manylots of masks just to sell them withfor a higher price forto those who need ithem. There are some people who buy lots of masks just to sell them for a higher price to those who need them.

There are even some people who buy so many masks only to sell them withat a higher price forto those who need it. There are even some people who buy many masks only to sell them at a higher price to those who need it.

"even," "only to" are for emphasis.

There are some people who buy so many masks to sell them withback for a higher price forto those who need it. There are some people who buy so many masks to sell them back for a higher price to those who need it.

There are some people who buy so manylots of masks just to sell them withat higher price fors to those who need ithem. There are some people who buy lots of masks just to sell them at higher prices to those who need them.

Some people call this "scalping." For example, "Some people are scalping surgical masks" means the same thing as "some people are buying lots of surgical masks and selling them at higher prices." It is mainly used for things like tickets or limited-edition products, but it can be used in a situation like this too.

PuttWearing a surgical masks is one unique aspect of Japanese unique cultures, butand it’s really useful not to catchto prevent the spread of viruses. Wearing surgical masks is one unique aspect of Japanese culture, and it’s really useful to prevent the spread of viruses.

Or "is one of Japan's unique cultural practices"

PuttNot only is wearing a surgical mask is one unique aspect of Japanese unique cultures, but it’s really useful not to catchit also helps prevent the spread of viruses. Not only is wearing a surgical mask one unique aspect of Japanese culture, it also helps prevent the spread of viruses.

The other correction is fine. This is another option. "Not only...but also" can be a good way to emphasize something you want to say, but I don't think it should be used too often.

Putting on a surgical mask is one of Japanesthe unique culturesstoms of Japan, but it’s really useful in order to not to catch viruses. Putting on a surgical mask is one of the unique customs of Japan, but it’s really useful in order to not catch viruses.

Putting on a surgical mask is one of the features of unique Japanese unique cultures, butand it’s really useful as to not to catch viruses. Putting on a surgical mask is one of the features of unique Japanese culture, and it’s really useful as to not catch viruses.

This phrasing sounds more natural. Your original sentence doesn't work right. Sorry :(

Imagine that you’re takriding a crowded train, and someone coughs andor sneezes without covering his/ or her mouth. Imagine that you’re riding a crowded train and someone coughs or sneezes without covering his or her mouth.

"Riding" sounds more natural to me.

The thing we’re most worried about the most isis being in close contacting with people without puttho don't have /aren't wearing masks. The thing we’re most worried about is being in close contact with people who don't have /aren't wearing masks.

Sounds more natural

The thing we’re most worried about the most isis being in close contacting with people without puttho aren't wearing masks. The thing we’re most worried about is being in close contact with people who aren't wearing masks.

Or if you mean that you are worried because you don't have any masks to wear, you can write: The thing we're most worried about is not being able to wear and mask while in close contact with other people.

The thing we’re worried about the most is making contacting with people without puttho are not wearing masks. The thing we’re worried about the most is making contact with people who are not wearing masks.

"contacting people" usually means 連絡する。 Not 接触する。 Making contact with people without wearing masks could be interpreted as マスクをしてない私たちは他の人と接触する。 やばいですね。

The thing we’re worried about the most is contacting people without putting on masks. The thing we’re worried about the most is contacting people without putting on masks.

I really hope the coronavirus finishepandemic ends as soon as possible. I really hope the coronavirus pandemic ends as soon as possible.

I really hope the coronavirus finishes as soon as possiblepandemic ends as soon. I really hope the coronavirus pandemic ends as soon.

"as possible" sounds like it is someone's choice/action. For example, "Please call me as soon as possible."

I really hope the coronavirus finisheends as soon as possible. I really hope the coronavirus ends as soon as possible.

Suggestion: I really hope this coronavirus emergency comes to an end as soon as possible.

I really hope the coronavirus finisheepidemic ends as soon as possible. I really hope the coronavirus epidemic ends as soon as possible.

"Finishes" would be used if there was some task at hand, but when talking about some event, you use "to end". "I finished the task" "The task ended" :)