Another Story Retelling of The Little Mermaid

Or, "A Retelling of the Story of The Little Mermaid. When an author writes a new story based on a classic story or a fairy tale, we call that a "retelling".

Long ago, a king and his servants were enjoying fishing on the sea, twhen they heard someone's singing in thea beautiful voice.

"when" combines the meanings of "then" and "as" and I think that's the meaning you're looking for. Like Brambsie said, "then" means that one action is over. "as" means that two actions are going at the same time. "when" used like this means that one action is going on and while that action is still happening, a second action begins. So, the sentence as I rewrote it means that they're out fishing on the sea, and then while they're still fishing, they begin to hear a voice singing.

They found three mermaid girls. The king spoke to them, and then the girls smiled and wentswam up near to the ship.

Technically, "then" is not a conjunction like "and" and "but" are. Native speakers use it like one all the time, but usually only when the subjects are the same. So, "He went to the store, then I called him" sounds weird, but "He went to the store, then called me" sounds perfectly natural.

In your sentence here, the subjects are not the same, so using "then" sounds funny. You can fix it just by adding an "and" or "so" before "then", or by replacing "then" with "and" or "so". For example, "The king spoke to them, so the girls smiled and went near the ship".

I changed "went" to "swam up" because "went" sounds weird replacing flying or swimming actions.

A servant whispered to the king that he had heard a story thabout someone who ate mermaid's meat and the person became immortal.

Do you mean that the servant heard a story about a particular person who ate mermaid meat and then became immortal, or that the servant knew a rumor/legend that anyone who eats mermaid meat will become immortal? Both work. The correction I gave says the first one. Here is how you would say the second meaning:

A servant whispered to the king that he had heard tell that if someone ate mermaid meat, that person would become immortal.

The king wais excited andthe king, so he commanded the cook to cook the weakened mermaids right away.

SAround her neck, she wore a beautiful pendant shining rainbow colors around her neckthat shone with all the colors of the rainbow.

Other options:

"Around her neck, she wore a beautiful pendant that shone in rainbow colors"

"She wore a beautiful pendant that shone rainbow colors around her neck", but that leaves it unclear whether the necklace is around her neck or the rainbow colors are around her neck haha.

"She wore a beautiful, shining, rainbow pendant around her neck" This one just means the pendant is shiny and rainbow colored, not that the pendant is shining in rainbow colors.

When the king woke up after he enjoyed aning the most amazing feast he had never eaten, he felt something different about his body.

He tried cutting his fingertip with a knife, then he saw his finger returning quickly to its original state.

See my previous comment about "then" :)

An even better way to say this would be "He tried cutting his fingertip with a knife and saw that his finger quickly returned to its original state".

He realized he had becaome immortal.

We use the past perfect tense when we're referring to something that happened BEFORE some point in the past. He realized something, so "realize" is past tense, but BEFORE that, he became immortal, so "become" is past perfect - "had become". I hope that makes sense - let me know if it doesn't and you'd like a better explanation :)

Then, she gazed atinto his eyes and whispered in a beautiful voice, “Avenge my sisters.

The A in avenge should be capitalized (I checked a grammar reference haha).

"Avenge" doesn't quite make sense, though. The girl is avenging her dead sisters by killing the king, but the king is not avenging them by dying. I think what you mean is "Pay for what you did to my sisters. Die". But... that's really wordy and doesn't sound as cool as what you wrote haha. "I am here to avenge my sisters" or "I have come to avenge my sisters" would both work, too.

The girl took off athe rainbow pendant from around her neck and the pendant turned a rainbow sword oin her palm.

The girl smiled at the witch and, put the pendant, which had turned back from the sword on, into the witch's palm, and then she didoved into the sea.

You can't have more than one "and" conjunction in a sentence. When more than two things happen, you just separate the events with a comma and a put one last "and" at the end - just like listing objects.

She became like marine snow and disappeared.

I don't know what you mean by "marine snow" here. Do you mean literal marine snow, the stuff that falls through the layers of the ocean? Or do you mean this poetically, like snow falling over the ocean?

The king was covered in black blood and had died.

Again, this is past perfect because he died before the maids found him, before they started screaming.

If you mean that he is actively dying in this sentence, it should be "The king, covered in black blood, died" or "The king died, covered in black blood".

Ever since, people have passed down athis legend.:

I'm sorry, this' is very long.

I tried to shorten it, but it's still long😓

Long ago, a king and his servants were enjoying fishing on the sea, then as they heard someone's singing in thewith a beautiful voice.

When two actions are going on at once, I would say "as" instead of "then". Saying "then" basically means that you're moving on to the next action, because the first one is over.

They found three mermaid girls. The king spoke to them, then the girls smiled and went near the ship.

You forgot a space between the two sentences :)

The men on the ship and the mermaid girls enjoyed an amusing conversation.

However, these men set a trap and caught, luring the mermaids onto the ship.

To "lure someone" means to "tempt someone to to do something or go somewhere". Did you mean this?
I wouldn't say "caught onto the ship". If you want to say they caught them, I would say:
However, these men set a trap, caught the mermaids, and brought them onto the ship.

A servant whispered to the king that he heard a story that someone ate mermaid's meat and, causing the person became immortal.

When the king woke up after he enjoyeding an amazing feast he had never eaten, he felt something different about his body.

"after enjoying" sounds smoother than "after he enjoyed".
Another example of a similar sentence:
The boy took a nap after playing in the park.

Also, did you mean to say "he had never eaten"? That means he did NOT eat the meal.

He tried cutting his fingertip with a knife, then he saw his finger returninged quickly to its original state.

A servant told the king about athe beautiful girl.

"A girl" means anyone in general. Since you are referring to the girl before, say "the girl"

Then, she gazed atinto his eyes and whispered in a beautiful voice, “Aavenge my sisters.

The girl took off athe rainbow pendant from her neck and the pendant turned a rainbow sword on her palm.

The king couldn't move , although he tried.

The girl smiled at the witch, and put the pendant turned back from the sword on the witch's palm, and then she dioved into the sea.

She became like marine snow and disappeared.

From his head to his toes, there were countless wounds, as though. It looked as if every internal part ruptured in his body, made himof his body had ruptured, making it unclear who he was.

Not only the king, but other servants, includeding the cook, were killed in the same way.

They were all the peopleEveryone who was killed was involved in the matter of the mermaids killed in the castle.

This sounds smoother.

Ever since, people have passed down athis legend.

I'm sorry, this' is very long.

Generally, I wouldn't contract "this is" to "this's".

Long ago, a king and his servants were enjoying fishing on the sea, then they heard someone's singing in thewith a beautiful voice.¶

The men on the ship and the mermaid girls enjoyed an amusing conversation.

The girl took off a rainbow pendant from her neck and the pendant turned a rainbow sword on her palm.

From his head to his toes, countless wounds, as though every internal part ruptured in his bodyof his body had ruptured, made himit unclear who he was.

Not only the king but other servants, includeding the cook, were killed in the same way.