I've seen the number one spelled "youn" and "en". What's the difference?

Yeah, you have en, youn, and yon
The only one you can most comfortably use before a noun (modifying a noun) is YON.
example:
Yo gen yon pitit → They have one child.
Ou gen sèlman yon chans → You only have one chance.

You use "EN" when counting
example:
en - de- twa
one-two-three

"YOUN" does not usually modify a noun.
for example, we don't say YOUN liv, we say YON liv (one book)
We use YOUN instead of YON at the end of sentences, and as an object pronoun.
example:
Mwen rete youn.
I have one left.
You cannot say Mwen rete YON or Mwen rete EN.

Mwen va chwazi youn ladan yo.
I will choose one of them.
so, you cannot use EN or YON here.

So, you'll say:
Ban m youn.
Give me one.
or 
Ban m yon pòm.
Give me an/one apple.

YOUN is also used in the expressions:
Youn apre lòt → one after the another
Youn dèyè lòt → one after the other
youn sou lòt → one on top of the other
youn pa youn → one by one
youn ou lòt → one or the other
youn lòt → one another
youn di lòt → one tell the other
youn te ede lòt → one helped the other, they helped each other
etc...

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words

one

one → en (when you are counting), youn, yon, len

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'has nothing to do' as in One has nothing to do with other. ? (thanks)

has nothing to do with (unrelated?) → pa gen anyen pou wè avèk

One has nothing to do with the other
Youn pa gen anyen pou wè avèk lòt.

This does not concern you.  This has nothing to do with you.
Sa pa konsènen w.  Sa pa gen anyen pou wè avè w.

each other, one and the other

each other → youn lòt
one and the other → youn ak lòt

1. They held each other's hand.
    Youn kenbe men lòt.

2. They're criticising each other.
    Youn ap kritike lòt.

3. They don't respect each other.
    Youn pa respekte lòt.

4. We love each other.  We respect each other.
    Nou youn renmen lòt.  Nou youn respekte lòt.

5. De fim sa yo se menm bagay la.  Si ou gade youn, ou pa bezwen gade lòt la.
    These two movies are one in the same.  If you watch one, you don't need to watch the other.

6. They were walking together.  Then one said to the other,  "Let us rest here for the night."
    Yo t'ap mache ansanm.  Epi youn di lòt la, "An nou repoze la a aswè a."

how do you say one

one - en, youn, yon grenn

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We are united

united → ini, fè youn

We are united
Nou ini
or
Nou fè youn

We were united like brothers and sisters
Nou te ini tankou frè ak sè.
or
Nou te fè youn tankou frè ak sè.

They are united together.
Yo ini ansanm
or
Yo fè youn ansanm

We are united in Christ.
Nou ini nan Kris.
or
Nou fè youn nan Kris.

Only choose one.

Jis chwazi youn.

or

Chwazi youn sèlman.

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The many ways to say "ONE" in Haitian Creole

one: youn, en, yon sèl, yon grenn, yon

yon

one car → yon machin

one house → yon kay

one life → yon lavi

Yon sèl

Only one God → yon sèl Bondye

Only one father → yon sèl papa

Only one friend → yon sèl zanmi

Yon grenn

Just one shoe → yon grenn soulye

Just one eye → yon grenn je

Just one child → yon grenn timoun

Just one single child → yon sèl grenn timoun

En

Used when counting numbers as in: 1, 21, 31, 41, etc...

Youn does not modify the noun.

A car

Youn machin (incorrect)

Yon machin (correct)

Youn comes after the verb or at the end of a sentence.

I need one. → Mwen bezwen youn.

Give me one. → Ban'm youn.

I got one, thanks! → Mwen jwenn youn, mèsi!

how to say "love one another" or others

Love one another. - Se pou nou youn renmen lòt.

Love others. - Renmen pwochen ou.

Love your neighbor. - Renmen pwochen ou.

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How do you say, Respect others?

Respect others - Respekte lòt.

Respect others. - Respekte pwochen ou.

Respect one another. - Respekte nou youn lòt.

Respect your brothers and sisters. - Respekte frè w ak sè w.

Respect yourself. - Respekte tèt ou.

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one

one - en (when you mean 1, 2, 3, ...)

one - youn (when you mean, "one person")


one of them - youn nan yo

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We are one in the Spirit, We are one in the Lord, and we pray that all unity will one day be restored and they'll know we are Christians by our love. All Praise to the Father, from whom all blessings come, All Praise to Jesus Christ, God's only Son.

We are one in the Spirit 

Nou tout fè youn nan Lespri Sen an.

We are one in the Lord.

Nou tout fè youn nan Bondye.

And we pray that all unity will one day be restored.
E nou priye ke yon jou inite sa va restore.

And they'll know we are Christians by our love.
E ya va konnen ke nou se Kretyen pa amou nou.

All praise to the Father, from whom all blessings come.
Tout glwa pou Bondye ki li sèl bay benediksyon.

All praise to Jesus Christ, God's only Son.
Tout glwa pou Jezi Kri, sèl Pitit Bondye a.

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how would you say one by one

one by one - grenn pa grenn, or youn pa youn

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