When I was 18 years old, I happened to watched the film "Apocalypse Now" by Francis Ford Coppola infrom 1979.

Saying 'I happened to...' implies you did it by accident. Both are grammatically correct, but I'm assuming you watched the movie intentionally.

It was an awesome movie which showed us about manymany of the contradictions in the Vietnam War, which some critics decidemed as the only one war that the USA lost.

'deemed' feels more appropriate here, since we're talking about the critics' opinions on the movie. They didn't decide anything.

I still remember someparts of thate movie still now.

Harrison Ford appeared / acted in that movie when he was relatively young.

Helicopter troops attacked a Vietnam village and killed all peopleveryone in the village including men, women and children, just because the captain of the troopstroops' captain wanted to surf on the village coast waves.

When choppers were attacking from the sky, they played Wagner's Die Walküre, Act III - Walkürenritt aloud wion the large speakers attached to the choppers as well asnext to the missiles.

https://www.youtube.com/watch?v=Wk7rl586GdQ ¶
The main character, who was on thea secret mission to assassinate another US army captain who became thea king and was worshipped by the native Vietnamees,se. He wanted to say something to the captain about hithe other US army captain's cruelty and immorality, then he said, "Some day, this war will come to its end."

I somehow understood that English sentence at that time.

Two Playmates from the magagine Playboy came to the Vetname stage to consolePlayboy magazine came to the stage in the American military base to entertain the US soldgiers.

However, the show turned out to be a mess, because many soldgiers suddenly became homesick and panicked, so the Playmates managehad to escape from the stage by Con a chopper.

A few soldgiers hung to the legfeet of the chopper, wanting to go home, but they droppedfell off from the high sky in the end.

Two or three soldgiers made searchwere roaming in the forest, when they met a wild tiger all of a sudden.

This wasese were the most easily understood Englighsh sentences for me in the movie.

I learned the four -letter word for the first time, and learned its usagehow to use it as well.

A native English speaker could call an animal with that four-letter (actually seven-letter) adjective, even when they were animal was not really mating.

When I was 18 years old, I happened to watch the film "Apocalypse Now" by Francis Ford Coppola released in 1979.

It was an awesome movie which showed us about many contradictions in the Vietnam War, which some critics decidedname as the only one war that the USA lost.

I still remember some parts of that movie still now.

Helicopter troops attacked a Vietnam village and killed all people in the village including men, women, and children, just because the captain of the troops wanted to surf on the village coast waves.

https://www.youtube.com/watch?v=Wk7rl586GdQ ¶
The main character, who was on the secret mission to assassinate another US army captain who became the king and worshipped by the native Vietnameese, wanted to say something to the captain about his cruelty and immorality, then he said, "Some day, this war will come to its end."

Two Playmates from the magagzine Playboy came to the Vietnamese stage to console the US soldgiers.

However, the show turned out to be a mess, because many soldgiers suddenly became homesick and panicked, so the Playmates managed to escaped from the stage by Cchopper.

A few soldgiers hung to the leg of the chopper, wanting to go home, but they dropped from the high sky in the end.

Two or three soldgiers madwere searching in thea forest, when they met a wild tiger all of a sudden.

This was the most easily understood Engligsh for me in the movie.

lol

When I was 18 years old, I happened to watch the film "Apocalypse Now" by Francis Ford Coppola infrom 1979.

"in 1979" implies you saw the movie in 1979

It was an awesome movie which showed us about manymany of the contradictions in( regarding / about ) the Vietnam War, which some critics decidedexperts see as the only one war that the USA has ever lost.

I still remember someparts of that movie still now.

Harrison Ford appeared in ( that movie / it ) when he was relatively young.

Helicopter troops attacked a Vietnamese village and killed all the people in ithe village including men, women and children, just because the captain of the troops wanted to surf on the village coast waves.

When choppers were attacking from the sky, they played Wagner's Die Walküre, Act III - Walkürenritt aloud with large speakers attached to the choppers as well as missiles.

https://www.youtube.com/watch?v=Wk7rl586GdQ ¶
The main character, who was on thea secret mission to assasinate another US army captain whoand later became the king andto be worshipped by the native Vietnameese, wanted to say something to the captain about his cruelty and immoral, tity. Then he said, "Some day, this war will come to its end."

I somehow understood that English at thate time.

Two Pplaymates from the magagzine Playboy came to the Vietname stagese battlefield to console the US soldgiers.

I think you mean battlefield? Stage refers to something in a theatre or show.

However, the showbattle turned out to be a mess, because many soldgiers suddenly became homesick and panicked, so the Pplaymates managed to escape from the stage by Chbattle by helicoppter.

A few soldgiers hung to the leg of the chophelicopter, wanting to go home, but they dropped from the high sky in the end they fell.

Two or three soldgiers made search in theed a forest, when they suddenly met a wild tiger all of a sudden.

This was the most easilyeasiest to understooand Engligsh for me in the movie.

A native English speaker could call an animal with that four-letter (actually seven-letter) adjective, even whenif they were not really mating.

It was an awesome movie which showed us abouthighlighted many contradictions in the Vietnam War, which some critics decided ashave deemed the only one war that the USA lost.

"critics" is usually paired with "deemed", so I wanted to show you this combo:

Andromeda is <deemed> a failure by many <critics> and...
<Critics Deemed> Them “The Most Miraculous Works of Modern Art America Has Produced”
<Critics deem> 'Suicide Squad' a 'disappointing disaster'

https://www.latimes.com/entertainment/herocomplex/la-et-hc-suicide-squad-critic-roundup-20160802-snap-htmlstory.html

I still remember some parts of that movie still now.

The main character, who was on thea secret mission to assassinate another US a.S. Army captain who had becaome thea king and was worshipped by the native Vietnameese, wanted to say something to the captain, then he said, "Some day, this war will come to itsan end."

It's safe to assume they have more than one mission in their careers, so I switched it to "a"
assassin - person
assassinate - action

Two Playmates from the magagzine Playboy came to the Vietname stage to consolemfort the US soldgiers.

I feel comfort is more suitable here because they provide "comfort" if you catch my drift :-)

However, the show turned out to be a mess, because many soldgiers suddenly became homesick and panicked, so the Playmates managed to escape from the stage by Cchopper.

This turned 180....

A few soldgiers hung to the leg of the chop, wanting to go home, but they dropped from the high sky in the end(eventually) fell from a great height.

in the end - eventually
fell - implies a person was initially in control, but lost their balance/strength, resulting in an unintentional fall

Two or three soldgiers madwere searching in the forest, when they met a wild tiger all of a suddensuddenly encountered a wild tiger.

Fucking tiger!

A native English speaker could call an animal with that adjectivemight use that adjective to describe an animal, even whenthough they we're not really fucking it.

This is pretty blunt, so it might be a good idea to replace the end with "engaging in the act"