S. R. Faruqi:

For the beloved to be intelligent, or to be quick/sharp in understanding the lover's intention-- this theme is not new. Mir has created two additional excellences in it. One is that the beloved's quickness of mind and her innate brightness are supported by the Unseen, or bestowed by the Lord. It's as though like beauty, acuity and intelligence too are eternally fated for her.

The second point is that when the moment the lip moves she arrives at the depth of the idea, then what is the delay? Why does she not understand the lover's/speaker's intention? The answer is that because of the oppression of her beauty, or because of agitation and unconsciousness, or because of self-lessness, in her presence he doesn't have the strength/boldness to open his lips. If the lover opened his mouth, then the beloved would at once understand the idea.

In the light of this meaning, the second line speaks in the past tense, about a future purpose (which is common in Urdu; for example, agar vuh aayaa to aap se .zaruur milegaa ). In fact the verb in the second line is expressing a longing/grief: 'if the lips could move [hil sakte] even a little bit, then she would at once understand [samajh letaa] my idea (that is, my true intention)'.

Here baat kii tah paa jaanaa is very fine, because in it is a suggestion that the real idea (a request for union, or an expression of passion) will not be said clearly and openly, it will be conveyed in signs and implications.

And ;zakaa meaning 'acuteness, sagacity' is an interesting word, for from the same root comes ;zakaa meaning 'sun'. To be able to 'find out the depth of' an idea, to 'shed light on' an affair-- here, to read ;zakaa as 'sun' is also fine. Then, taa))iid udhar kii hai too is superbly colloquial.

FWP:

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MOTIFS
NAMES
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The beloved's mental quickness and sharpness no doubt get support from Over There, from Beyond, from the Unseen. A faint preliminary quivering of the lover's lip(s) is all it takes for her to instantly grasp the whole of what hasn't been said. But here there are two different possibilities. One is that the lover is just barely beginning to speak (or trying to begin), and she would grasp the whole of what he is planning to say, including its underlying significance. The other is that the lover does not intend to speak, but his lip involuntarily quivers, and even from that tiny little sign she intuits the whole of what he would not wish to say, including its underlying significance. (And why should she not, since she has aid from That Side?)

Note for grammar fans: For ho;N;Th hile there are two possibilities: it could be a future subjunctive in the singular ('if the lip would move'), or else a perfect in the plural ('if the lips moved'), which is often colloquially used, as SRF notes, to express a future subjunctive meaning. SRF also seems to suggest a contrafactual possibility, which I don't quite follow.

Note for grammar fans: For discussion of paa jaave , see the grammar note in {1781,1}.