I sometimes play the instruments at work.

The "the" would require that you had already specified the instruments before and now you were bringing them up again.

I usually play the stringed instruments.

The "the" feels less weird here than in the sentence before, but the sentence still feels more natural without it.

But, Otherone day, our co-worker took day off, who plays the piano, took the day off.

"One day" feels better when taking about a random day. Think of "But our co-worker took the day off" as the main phrase, and now you want to fit other informations in it. The time expression is perfectly positioned, but it is better if you put the sentence "who plays the piano" right after the word which it refers to.

"Who will plays the piano?" crossed the our heads.

"Who plays the piano?" is more like "Who here has the ability to play the piano?" and "Who will play the piano?" fits more if they are asking themselves who would play the piano, if the one with this "job" took the day off.

I translated your native text and, given the context, I felt like the second option was closer. But sorry if I misunderstood.

A few of my coworkers were backing away like they were doing thea backstroke.

The whole chunk is "a few of". Try to fit it as a whole in sentences.

In the end, a new a man's employee played the piano for us.

Keep it simple, especially in the beginning of your learning: article - adjective - noun

I sometimes play themusical instruments at work.

"the instruments" would be referring to some specific previously introduced set of instruments. Since this is your first time introducing them, it's more natural to say "play musical instruments"

I usually play the stringed instruments.

"the" is ok here, I think because you're selecting some instruments from the set of instruments you've just introduced.

But, Othe other day, our co-worker took day off who plays the piano took a day off.

More natural order, since "who plays the piano" is sort of describing the coworker, it's more natural to put it closer to the noun compared to the action.

"Who plays(can play / will play) the piano?" crossed the our heads.

More natural phrasing.

"can play" would be if you're looking at each other and asking who has the ability to play piano. "will play" has more of an implication that the piano needs to be played and you're looking for a volunteer.

AndThen we made eye contact.

"And" at the start of a sentence is often considered poor style.

Our eyes were looking all over the place—we were so nervous.

A few of my coworkers were backing away like they were doing the backstroke.

In the end, a new a man's(male) employee played the piano for us.

"male" is the adjective form to use here, except you don't need to specify male here really - it's pretty self evident from the next sentence where you use "he" as a pronoun.

From that day on , he was secretly called "Piano Man".

Commas have a space after them, but not before them.

There's a famous song from the 1970s by Billy Joel called Piano Man ( https://www.youtube.com/watch?v=gxEPV4kolz0 ). Was the nickname referencing that?

I sometimes play the instruments at work.

I usually play the stringed instruments.

But, O the other day, our co-worker took day off who plays the piano took a day off.

"Who is going to plays the piano now?" crossed the our heaminds.

Our eyes were all over the place—we were so nervouslooking around nervously, as if they were swimming.

This provides a backdrop for the swimming-related metaphors you are going to use, while also better matching your Japanese version.

A few of my coworkers were backing away like they were doing the backstroke.

In the end, new a man'sle employee who recently joined played the piano for us.

From that day on , he was secretly called the "Piano Man".

I sometimes play thean instruments at work.

I usually play the stringed instruments.

As another user has said, it might be better to say "string instruments", but both are fine.

But, Othe other day, our co-worker took day off whowho usually plays the piano took the day off.

"Our co-worker took (the) day off who play(s) the piano" sounds like the day is playing the piano, which doesn't make sense.

The "usually" isn't necessary, but makes this more natural, I think. This is a routine that usually happens normally but, as your story shows, can sometimes be broken.

"Who's going to plays the piano now?" crossed the our heaminds.

Alternative phrasing: "So then we thought, "now who's going to play the piano?""
The "now" can be thought of less like a time frame and more like "because of this new situation". For example, "I just lost my keys, how am I going to get in now?", or "Now that I'm finished with work, I can relax."

It sounds awkward to use "the" and a possessive together, like "the our" or "the my". The "the" should be dropped in this case -- possessives like "my" or "our" sort of work like their own versions of "the".

A few of my coworkers were backing away like they were doing the backstroke.

Unfortunately, I don't know what you mean by "backstroke".

In the end, a new a man'smale employee played the piano for us.

Adjectives like "new" will never go before an article like "a" or "the". Same with possessives like "my" or "our". "I took my fluffy dog to a pretty park."

"Man's _____" means a _____ which belongs to or is associated with a man.

From my experience, English speakers don't commonly specify gender using adjectives unless if the gender is so important to the context that it MUST be specified. So we'd probably just say, "In the end, a newer employee played the piano for us." But if you really want to say that the person is a man or a woman using an adjective, "male" or "female" would work. Keep in mind, though, that these words have a slight medical tone to them.

From that day on , he was secretly called "Piano Man".

I sometimes play the an instrument (instruments) at work.

If more than one instrument --> instruments

I usually play the stringed instruments.

"string instruments", "wind" and "percussion" are different classes of instruments. "stringed" is grammatically OK but not natural.

But, Othe other day, ourmy co-worker took day off who plays the piano took the day off.

誰 who 動詞 is the pattern.
example:
the man who ate ramen
the woman who works in the building next door

The thought "Who plays the piano?" crossed the our heaminds.

"cross my mind" is an expression used to express a sudden or fleeting idea

And wWe made eye contact.

It's unusual to start a sentence with "and". Instead, remove "and" OR add "and we made eye contact" to the previous sentence (crossed our minds and we made eye contact)

In the end, a new a man'smale employee played the piano for us.

I sometimes play the instruments at work.

I usually play the stringed instruments.

But, O the other day, ourmy co-worker took day off who plays the piano took the day off.

"Who plays the piano?" crossed the our heaminds.

A few of my coworkers were backing away like they were doing the backstroke.

In the end, a new a man's employee played the piano for us.

From that day on , he was secretly called "Piano Man".

I sometimes play the instruments at work.

I'm not sure what your job is, but if it is a workplace centered around playing instruments, the sentence is correct. If it doesn't, and you just play instruments there unrelated to the work being done, then you would remove "the" (I sometimes play instruments at work).

I usually play the stringed instruments.

Same feedback as previous sentence about when to remove "the".

But, O the other day, our co-worker took day off who play the pianowho plays piano took the day off.

"Who is going to plays the piano?" crossed the our heaminds.

It's written as "crossed our minds" ("to cross one's mind").

And then we made eye contact.

A few of my coworkers were backinged away like they were doing the backstroke.

In the end, new a man'sa man who had just recently became an employee played the piano for us.

From that day on , he was secretly called "Piano Man".