Why are these Sanskrit words in the nominative case

Question

I'm studying the Sanskrit mantra that starts with asato ma:

असतो मा सद् गमय        asato mā sad gamaya 
तमसो मा ज्योतिर्गमय       tamaso mā jyotirgamaya
मृत्योर्मा अमृतं गमय        mṛtyormā amṛtaṃ gamaya

The meaning of the first two lines is "lead from the unreal to the real. Lead me from darkness to light".

Because of the 'to' particle in English, it would seem to be that the dative case should be used.

But the words सद् and ज्योतिर् are being used.

I think that after having broken down the sandhi, we are talking about सत् and ज्योतिः

Both of these are nominative. Why not dative?

Am I missing something?

Answer 1

They are not nominative. Both of them are neuter nouns, which means that nominative and accusative look the same. In fact, they are in accusative case and you might want to call it "accusative of direction" or "goal of movement". Same holds true for amṛtam.

Answer 2

There's a deeper reason for selecting the विभक्तिः (case-ending) for each of the words. The basis for this is Panini's sutra (पाणिनेः सूत्रम्)

1-4-52 गतिबुद्धिप्रत्यवसानार्थशब्दकर्माकर्मकाणामणि कर्ता स णौ

It mandates that when any of the verbal roots (धातवः) in this sutram (or other roots which carry the same meaning as those in the rule) is used in the causative sense (ण्यन्तावस्था), then agent of action (कर्ता) in the non-causative sentence (अण्यन्तावस्था) shall become the object (कर्म) in causative sentence (ण्यन्तावस्था). The example given to demonstrate application of this rule is

शत्रूनगमयत् स्वर्गं वेदार्थं स्वानवेदयत् ।
आशयच्चामृतं देवान् वेदम् अध्यापयद् विधिम् ।
आसयत् सलिले पृथ्वीं यः स श्रीहरिर्मे गतिः ।।

The closest translation for the above verse in English is:

He sent the enemies to heaven, he made his own people learn the meaning of vedas
He made the devas drink nectar, he taught (i.e made learn) Brahma veda,
He placed the earth on ocean, such a Hari is my refuge.

Each of the sentences in the verse has a causative verb e.g अगमयत्, अवेदयत् etc. In the non-causative form, each sentence can be written as

शत्रवः स्वर्गम् अगच्छन् - Enemies went to heaven - verb गम् indicating movement
स्वे वेदार्थम् अविदुः - His own people learned the meaning of vedas - verb विद् indicating बुद्धिः or learning
देवाः अमृतम् आश्नन् - The devas drank nectar - verb अश् indicating eating 
विधिः वेदम् अध्यैत - Lord Brahma studied veda - verb इङ् indicating study 
पृथ्वी सलिले आस्त - Earth stood (or rested) on the ocean - verb आस, an intransitive verb (अकर्मकधातुः)

Each of the verbs used in the above example are directly called out in the sutram, or have meanings of one of the verbs explicitly mentioned in it. Hence, in the causative construct, the agent of action in each of these sentences becomes the object.

Applying the same sutram to the verse

असतो मा सद् गमय 
तमसो मा ज्योतिर्गमय
मृत्योर्मा अमृतं गमय

Let us look at the non-causative sentences first.

अहम् असतः सद् गच्छानि - I may go from falsehood to truth
अहं तमसः ज्योतिः गच्छानि - I may go from darkness towards light
अहं मृत्योः अमृतं गच्छानि - I may go from death to immortality

Since the verb गम् (with meaning movement) is used in every sentence, the sutram (quoted above) mandates that the agent of action here (अहम्) shall become the object (कर्म) in the causative construction. Therefore, the sentences will be

मा (त्वम्) असतः सत् गमय - (You) lead me from falsehood to truth
मा (त्वं) तमसः ज्योतिः गमय - (You) lead me from darkness to light
मा (त्वं) मृत्योः अमृतं गमय - (You) lead me from death to immortality

One source of confusion is that the words in नपुंसकलिङ्गः (neuter-gender) सद् ज्योतिः अमृतम् all have the same forms in nominative and accusative:

सत्/सद् सती सन्ति
सत्/सद् सती सन्ति

ज्योतिः ज्योतिषी ज्योतींषि
ज्योतिः ज्योतिषी ज्योतींषि 

अमृतम् अमृते अमृतानि
अमृतम् अमृते अमृतानि