From Learn Na'vi Wiki
Because of Na'vi's ergative-accusative case structure, to speak even a simple sentence of Na'vi requires one to understand verb transitivity. The subject of an intransitive verb gets no case marking, but the subject of a transitive verb will get the agentive (ergative) ending, -(ì)l.
Na'vi verb roots may have one of three inherent transitivities:
- inherently transitive, as in the verb taron hunt
- ambivalent transitivity, as in the verb pey, which can mean either wait (intransitive) or await, wait for (transitive)
- inherently intransitive, as in the verb kä go
The transitivity of the ambivalent verbs must be determined from the case usage in the clause, oel pey trrit I'm waiting for the day or oe pey I'm waiting.
In general the verb transitivity will be clear from the semantics of the word itself (taron hunt is pretty obviously transitive), but there is still a substantial list of verbs for which this must be memorized (sngä'i begin, which is intransitive) (Transitivity).
A causative verb, with the ‹eyk› infix in pre-first position, is transitive.
Causative of Intransitives
When an intransitive verb takes the causative infix, the causee is in the accusative case.
- po kä neto she goes away becomes
- oel keykä pot neto I make her go away
Causative of Transitives
When a transitive verb takes the causative, the original object stays in the accusative case, and the causee takes the dative.
- Neytiril yerikit tolaron Neytiri hunted hexapede
- Eytukanìl Neytirir yerikit teykolaron Eytukan made Neytiri hunt a hexapede.
Another possible way to express the causee is with the adposition fa. This takes attention away from the causee a bit, focusing instead on the object and the causation.
- Eytukanìl fa Neytiri yerikit teykolaron Eytukan had a hexapede hunted by Neytiri. (Feb 17).
A reflexive verb, with the ‹äp› infix in pre-first position, is intransitive. Oe tsäpe'a I see myself; oe yäperur I'm washing myself. (RaN).
Na'vi verbs which are inherently transitive may be used in an intransitive construction, similar to what linguists call an antipassive. This is used when the direct object is suppressed as irrelevant. For example, oe taron I hunt is a general statement about one's activities, where what one is hunting in particular doesn't matter.
The antipassive should be distinguished from omission of a direct object which exists in the context. For example,
- Ngal ke tse'a txepit srak? Do you not see the fire?
- Oel tse'a I see (it).
Here the direct object is simply not mentioned, rather than suppressed entirely, so the verb is still counted as transitive.
Note: waiting on suomichris to get permission to release Frommerian email on this.
Verbal idioms composed of a noun with si do, make are intransitive. What would be thought of as the direct object in English will take the dative, ngaru seiyi irayo I thank you (various). 
The reflexive marker in these indicates the middle voice, as in win säpi hurry, from win si rush something.
The simple future tense has two sets of forms, the simple futures ‹ìy› and ‹ay›, and the s-futures ‹ìsy› and ‹asy›. The s-futures indicate a positive determination to make something happen rather than a simple prediction about the future, pìsyeng oe ngar I will tell you (forum thread).
Combined Tense and Aspect
Commands in Na'vi have no special infix. Positive commands are simply a verb stem, kä! kä! go! go! (from the film). The pronoun may also be stated explicitly, 'awpot set ftxey ayngal, (you) choose one now (ASG, Hunt Song).
A command may also use the infix ‹iv›. Frommer says, "at an earlier point in the history of the language there was probably a polite/familiar distinction (the -iv- form being the politer one), but that's no longer the case. They're used interchangeably. So to say "Go!" you can say either kivä or just kä."
Negative commands are not negated with the usual negative adverb ke, but rather use the word rä'ä, as in txopu rä’ä si don't be afraid (from the film), tsakem rä’ä sivi don't do that (action) (GMA).
The subjunctive is marked with the infix ‹iv›. It has quite a range of uses.
It is used to indicate wish, oeyä swizaw nìngay tivakuk let my arrow strike true (ASG, Hunt Song).
The verbal complement to a modal verb, such as zene must, tsun can, be able to, and new, want to, will take the subjunctive, as in ayngari zene hivum you must leave (BtS), oe new nìtxan ayngaru fyawivìntxu I want very much to guide you (AMFP), fmawn a tsun oe ayngaru tivìng news which I can give to you (AMFP).
This construction is a bit unusual for modal verbs, compared to many languages, but reflects the reduction of a complex sentence from which the subordinating conjunction has disappeared.
Known modal verbs:
fmi try, attempt new want sngä'i start, begin tsun can, be able zene must, have to
Purpose clauses using the conjunction fte (negative fteke) take the subjunctive, sawtute zera'u fte fol Kelutralti skiva'a the sky people are coming to destroy Hometree (BtS), fteke nìhawng livok to not get too close (NYT sound sample).
We do not yet know all the types of conditional sentences in Na'vi. At least some do use the subjunctive in the protasis, pxan livu txo nì'aw oe ngari / tsakrr nga Na'viru yomtìyìng only if I am worth of you / will you then feed the people (ASG, Hunt Song).
The conjunction tsnì that introduces some kinds of report clause which cause the verb to take the subjunctive, ätxäle si tsnì livu oheru Uniltaron I respectfully request the Dream Hunt (BtS). In Frommer's own notes to AMFP, he says of sìlpey hope, "optionally takes tsnì to mark the clause and requires -iv- in the subordinate verb(s))."
The evidential ‹ats› marks uncertainty or indirect knowledge (Evidential).