Have a look at my sentances, rutxe?

[Ngawng]

Kaltxì ma aytsmuk.

Na'vi
Attempted meaning


Leram ngaru ke sayi tsapxalute srak?
You're not going to say sorry?

Fteria le'fya nìNa'vi oe
I am studying Na'vi language

AyNa'vi-yä eana txok pelun? (now I'm sure that one is wrong....)
Why do the Na'vi have blue skin?

Ftang leru skxawng!
Stop being an idiot!

Tsakrr oe ramol hu oeyä 'eylan Kasì taweyk tsakrr lu tutéyä krr
Today I sang with my friend Cassie because today is Women's Day.

Yeah that last one was a little weird. If anyone wants to help, keep it simple rutxe...oe lu skxawng! 

[Unil Akawng]

Kaltxì ngaru, ma Nawma Ngawng!

Let's see:

Leram ngaru ke sayi tsapxalute srak?
You're not going to say sorry?

Here I'd go with Nga ke tsap'alute sìyi srak?, as the meaning of "going to" is captured by the immediate future infix <ìy>, in my opinion. Also, I'm not quite getting what you were aiming at with leram ngaru ("it seems", maybe?).

Fteria le'fya nìNa'vi oe
I am studying Na'vi language

An adjective leNa'vi will serve better than an adverb here, because it's attributed to a noun lì'fya. Otherwise, it's OK.
EDIT: Sorry, missed an important thing: case markings. Here, you need Fteria lì'fyat leNa'vi oel, because ftia is transitive here.

AyNa'vi-yä eana txok pelun? (now I'm sure that one is wrong....)
Why do the Na'vi have blue skin?

Here I'd use Na'viyä aysokx ean lu pelun?
Na'vi ("the People") probably don't need plural, while tokx (which is OK until we recieve an actual word for skin) does.  
Besides, there needs to be a verb to form a proper sentence, hence lu is used to attribute an adjective.

Ftang leru skxawng!
Stop being an idiot!

See no nits to pick here  . EDIT: But it appears I'm wrong. Please see below!

Tsakrr oe ramol hu oeyä 'eylan Kasì taweyk tsakrr lu tutéyä krr
Today I sang with my friend Cassie because today is Women's Day.

If you want "today", it's better to use fìtrr, methinks, as tsakrr means "that time". Also, talun ("for the reason") seems a better fit then taweyk ("based on the cause"), but otherwise it's fine!

Sìlpey oe tsnì fìtì'eyng tsìyun ngaru srung sivi!

[omängum fra'uti]

AyNa'vi-yä eana txok pelun? (now I'm sure that one is wrong....)
Why do the Na'vi have blue skin?

Here I'd use Na'viyä aysokx ean lu pelun?
Na'vi ("the People") probably don't need plural, while tokx (which is OK until we recieve an actual word for skin) does. 
Besides, there needs to be a verb to form a proper sentence, hence lu is used to attribute an adjective.

I'd agree that Na'vi likely doesn't need to be plural, as it seems like it's already all-encompasing.  However, different language treats agreement of plurality differently.  In some languages it would be proper to use the plural there, in others it would not and using it would be saying they each have multiple skins.  My gut feeling is Na'vi is actually the latter, but I'm not going to commit to that at this point.

But back on topic, just throwing lu in there is saying "Why are the Na'vi blue bodies?"  You'd need a case on body there.  Either dative (Why do the Na'vi have blue bodies?) or topic (About the Na'vi, why are bodies blue?)

Ftang leru skxawng!
Stop being an idiot!

See no nits to pick here  .

Well I'll pick one for you.   You have two verbs there shoved together, but nothing to show how they relate.  A simple fwa should do the trick (fwa = fì'u a)
Ftang fwa leru skxawng!

Aside from that, I agree with the rest of the comments.

[Unil Akawng]

But back on topic, just throwing lu in there is saying "Why are the Na'vi blue bodies?"  You'd need a case on body there. 

Probably I'm missing something, but why Na'viyä sokx (bodies of the Na'vi) will not work?

Well I'll pick one for you.   You have two verbs there shoved together, but nothing to show how they relate.  A simple fwa should do the trick (fwa = fì'u a)
Ftang fwa leru skxawng!

Ah! Indeed my solution was wrong. It's just grammatical patterns from my native tongue that continue to crop up in my Na'vi...

[Nawmaritie]

AyNa'vi-yä eana txok pelun? (now I'm sure that one is wrong....)
Why do the Na'vi have blue skin?

Here I'd use Na'viyä aysokx ean lu pelun?
Na'vi ("the People") probably don't need plural, while tokx (which is OK until we recieve an actual word for skin) does.  
Besides, there needs to be a verb to form a proper sentence, hence lu is used to attribute an adjective.

I'd agree that Na'vi likely doesn't need to be plural, as it seems like it's already all-encompasing.  However, different language treats agreement of plurality differently.  In some languages it would be proper to use the plural there, in others it would not and using it would be saying they each have multiple skins.  My gut feeling is Na'vi is actually the latter, but I'm not going to commit to that at this point.

But back on topic, just throwing lu in there is saying "Why are the Na'vi blue bodies?"  You'd need a case on body there.  Either dative (Why do the Na'vi have blue bodies?) or topic (About the Na'vi, why are bodies blue?)

Are you sure? Leaving the question word (pelun) aside one could form three sentences with the same basic meaning:

(1) Genitive: Na'viyä sokx lu ean - the bodies of the Na'vi are blue

(2) Dative: Lu Na'viru sokx aean - to the Na'vi are blue bodies -> The Na'vi have blue bodies

(3) Topic:  Na'viri sokx lu ean  - About the Na'vi: bodies are blue

So the construction (1) with the genitive for ,,Na'vi" should work. Does adding the question word ,,pelun" change that ?

[omängum fra'uti]

Eh, somehow I was so busy focusing on the plural aspect that I forgot the genitive on Na'vi.  (I have a very short memory.)  You are correct, any of those three would work fine.