The Tao begot one.
One begot two.
Two begot three.
And three begot the ten thousand things.
The ten thousand things carry yin and embrace yang.
They achieve harmony by combining these forces.
Men hate to be 'orphaned', 'widowed' or 'worthless',
But this is how kings and lords describe themselves.
For one gains by losing
And loses by gaining.
What others teach, I also teach, that is:
'A violent man will die a violent death.'
This is the essence of my teaching.
 Lau and the Ma wang tui text state, 'carry yin on their backs and wrap their arms around the yang'.
 Lau says: '... and are a blending of the generative forces of the two.'
 Lau uses 'solitary', 'desolate' and 'hapless'. The Ma wang tui text has 'orphaned, widowed and have no grain'.
 Lau says: 'Thus a thing is sometimes added to by being diminished or diminished by being added to.
 Both Lau and the Ma wang tui text say rather, '... will not come to a natural end', which is not quite the same thing.
 The Ma wang tui text has '... as the father of my studies', and Lau '... take this as my precept'.
Wang Pi (at least in Lynn's translation, I wouldn't know about the original unfortunately) again divides his commentary on the first section (excluding only the last three lines) into three sections as follows:
1. Although the myriad things exist in myriad forms, they all revert to the One. What is it that causes them all to ultimately become One? It is due to nothingness (wu). Because it is from nothingness that the One comes, the One can be called 'nothingness'. Because we already call it 'One', how can there not be a word for it? Because we have this word and because we also have the One, how can there not be two? Because we have the One and these two (the words 'one' and 'two'), this consequently gives birth to three. The numbers involved in the transition from nothingness to existence are thus all accounted for here. If one passes this point and keeps on going, any such path is not the course of Tao.(*)
(*) Fascinatingly here, we are reminded of Aleister Crowley's contention that 0 = 3 inasmuch as, if you have nothing, understand that there is nothing and then go on to symbolise that nothing you are already at a third order of things.
2. Therefore the myriad things are begotten, and I know the master controlling this. Although they have myriad forms, it is the fusion of the vital principles that makes One out of them all. each of the common folk has his or her own heart/mind and customs differ from state to state, yet any lord or prince who attains the One becomes master over them all. Since he becomes master thanks to the One, how could this One ecver be discarded?
3. The more one has, the further removed one is from the One, but being diminished gets one closer to it. When diminution reaches its limit, its ultimate value (the One) is attained. Since calling it 'the One' already brings us to three, how much less likely is it that someone rooted in something other than the One will ever get close to the Tao? The saying that one is 'augmented by being diminished and... diminished by being augmented' is not just empty talk!
Of the remaining three lines he says:Teaching others I do not force them to follow what I teach but help them to make use of the natural which I cite as the perfect principle, and that compliance with this means good fortune while opposition to it brings misfortune. Thus, as regards what people teach each other, if one opposes it, one will certainly bring misfortune upon oneself. Thus I also teach others in such a way as not to oppose them.
If one is dangerously bold, one will certainly not die a natural death...
Cheng Man-ch'ing, however, gets to nub of the matter. I shall base my own commentary around his, changing and/or augmenting his (which I'm not convinced his translator understood fully) to suit my own understanding. (Please bear with me!)
Because it is invariant and continuous like light, yang energy is represented as a solid line: ___. When yang energy peaks, yin energy arises, and, since yin is the embodiment of the yang, its manifestation in the discrete and intermittent, it is represented by the broken line _ _.
These lines are what inspired the words 'unity gives birth to duality".
In the I Ching, yin-yang is symbolised by these two lines one on top of the other, thus:
which is, of course trinity.
The broken yin line, too, is already a trinity in itself (line-space-line), and the solid yang line may also be regarded as a trinity in that it enlivens and 'fills' the three sections of the yin line.
When yin peaks, yang arises, and this is 'returning' and the cycle of continuous change.
This is the interplay of yin-yang, or 'the marriage of heaven and earth'.
If one adds another line, yin or yang, and above or below, to the above combination, one gets the six 'offspring' trigrams:
_ _ _ _ ___ _ _ ___ ___
_ _ ___ _ _ ___ _ _ ___
___ _ _ _ _ ___ ___ _ _
which, along with the mother and father trigrams
___ _ _
___ _ _
___ _ _
represent the 5 stages of change, fire, water, wood, metal and earth. These in combination (any trigram can combine with all eight of the others thus making 64 hexagrams, each of which can then either remain itself or change into any one of the 63 others through the mutation of lines moving from old yang to young yin or vice-versa, giving 4096 possible 'combinations') are the 'trinity giving birth to the ten thousand things' which then, in turn, reproduce.
This trinity is then also sometimes symbolised by a threefold broken line, _ _ _, and possible 'combination' at that point also becomes somewhat mind-boggling...
'The breath of heaven and earth blend and nature quickens', 'male and female unite and nature is impregnated'.
As Wang Pi says, stepping beyond this is to lose the path.
As regards 'all things are wrapped in yin and contain yang', yin makes up the external form of the yang that it embodies - that is contained within and enlivens it. The mutuality of this interplay between them - the yin enfolding the yang --->, and the yang indwelling the yin <---, can also be seen as a third element <--->.
'Their pulsating ch'is marry' is the circulating current of ch'i producing the one and the other energy by turns in what the I Ching calls the processes of 'transfer' and 'transformation'. According to these processes, all things are born of the energy of earth - are 'wrapped in yin' - and hence yin and softness are the dominant characteristic of earth.
'The stiff and the hard are the moribund ones' describes the situation where overabundant strength goes against the energy of the ch'i and must inevitably fail because of this. It results in 'untimely death'. The Shuo Wen, the earliest compiled Chinese dictionary from the 2nd. c. CE, defines 'proper behaviour' as 'the everyday household rules of good behaviour'.
Lao-tze is a proponent of yin and of softness, the exact opposite of overweening strength, and therefore 'chooses this' and teaches proper behaviour.
This last is a blend of my own reading and what I think is a slightly garbled version of Professor Cheng. For any errors in it, please forgive me.