I also could speak as ye do: if your soul were in my soul's stead, I could heap up words against you, and shake mine head at you.

I also could speak as ye do; If your soul were in my soul's stead, I could join words together against you, And shake my head at you.

I also could speak like you: and would God your soul were for my soul. I would comfort you also with words, and would wag my head over you.

I also, like you, might speak, If your soul were in my soul's stead. I might join against you with words, And nod at you with my head.

It would not be hard for me to say such things if your souls were in my soul's place; joining words together against you, and shaking my head at you:

I, too, can speak like you; and I also wish that your soul favored my soul. I would also comfort you with speeches and would wag my head over you.

I also could speak as ye do - In the same reproachful manner, and stringing together old proverbs and maxims as you have.
If your soul were in my soul's stead - If you were in my place. The idea is, that there is no difficulty in finding arguments to overwhelm the afflicted - a truth which most persons who have been unfortunate, have had opportunity to experience.
I could heap up words against you - Or, rather, "I could string together words against you." The idea is not that of heaping up, or accumulating; it is that of tying together, or uniting; and refers here to stringing together old maxims, saws, and proverbs, in the form of a set argument or discourse. The idea of Job is, that their discourses were nothing but ancient proverbs, thrown together, or strung along without regard to order, pertinency, or force. The Hebrew word used here (חבר châbar) means to bind, to bind together, to associate, to be confederate. It may be applied to friends - united in friendship; to nations - united in an alliance, etc. Gesenius supposes that it means here that he "would make a league with words against them;" but the above seems to be the more probable interpretation. The Septuagint renders it, "then I could insult you - ἐναλοῦμαι enaloumai - with words." Jerome (Vulgate) "I would console you with words, and move my head over you." The Chaldee is as the Hebrew - חבר châbar. Dr. Good renders it, "against you will I string together old sayings."
And shake mine head at you - An action common to all countries and ages, expressive of contempt, or of threatening; compare Jeremiah 18:16; Lamentations 2:15; Zephaniah 2:15; Matthew 27:39. So Lucretius ii. 1163:
Jamque caput quassans grandis suspirat ararat
Crebrius incassum magnum cecidisse laborem.
In like manner Virgil, Aeneid xii. 292:
Tum quassanos caput, haec effudit pectore dicta.
So, also, Homer, Odyssey ε e:
Κινήσας δὲ κάρη πρότι ὅν μυθήσατο Θυμόν.
Kinēsas de karē proti hon muthēsato thumon.
The meaning of Job here is, that be could as easily have expressed contempt, reproach, and scorn, as they did. It required no uncommon talent to do it, and he felt that he would have been fully sufficient for the task.

I also could speak - It is probably better to render some of these permissives or potential verbs literally in the future tense, as in the Hebrew: I also Will speak. Mr. Good has adopted this mode.
If your soul were in my soul's stead - If you were in my place, I also could quote many wise sayings that might tend to show that you were hypocrites and wicked men; but would this be fair? Even when I might not choose to go farther in assertion, I might shake my head by way of insinuation that there was much more behind, of which I did not choose to speak; but would this be right? That such sayings are in memory, is no proof that they were either made for me, or apply to my case.

I also could speak as ye [do]: if your (c) soul were in my soul's stead, I could heap up words against you, and (d) shake mine head at you.
(c) I would that you felt what I do.
(d) That is, mock at your misery, as you do at mine.

I also could speak as ye do,.... As big words, with as high a tone, with as stiff a neck, and as haughtily and loftily; or "ought I to speak as you do" (m)? that I ought not, nor would you think I ought, if you were in my case; or, being so, "would I speak as you do" (n)? no, I would not, my conscience would not suffer me:
if your soul were in my soul's stead; in the same afflicted state and condition, in the same distressed case and circumstances; not that he wished it, as some render the words, for a good man will not wish hurt to another; only he supposes this, as it was a case supposable, and not impossible to be a fact, some time or another, in this state of uncertainty and change; however it is right to put ourselves in the case of others in our own imagination, that so it may be considered in the proper point of view, that we may better judge how we should choose to be treated ourselves in such circumstances, and so teach us to do that to others as we would have done to ourselves:
I could heap up words against you; talk as fast as you to me, and run you down with a great torrent of words; Job had a great fluency, he talked a great deal in his afflicted, state, too much as his friends thought, who represent him as dealing in a multitude of words, and as a very talkative man, Job 8:2; and what could he have done, had he his health, and in prosperous circumstances as formerly? he could have brought many charges and accusations against them, as they had against him; or "would I heap up words against you?" or "ought I?" &c. (o); no, it would not be my duty, nor would I do it; humanity and good sense would never have allowed me to do it; but, on the contrary, I "would have joined myself with you", in a social, free, and familiar manner, in words (p), in a friendly meeting with you, so the words may be read and paraphrased; I would have come and paid you a visit, and sat down by you, and entered into a kind and compassionate conversation with you about your case and condition, and done all I could to comfort you; I would have framed and composed (as the word used signifies) a set discourse on purpose; I would have sought out all the acceptable words, and put them together in the best manner I could for you (q); had I the tongue of the learned, I would have made use of it, to have spoken a word in season to you:
and shake mine head at you; by way of scorn and derision, that is, he could have done it as well as they; shaking the head is used as a sign of contempt, Psalm 22:8; or "would I", or "ought I to shake my head at you" (r) if in my case? no, I would not; as I ought not, I would have scorned to have done it; or the sense may be, "I would have shook my head at you", in a way of pity, bemoaning lamenting, and, condoling your case (s); see Job 42:11.
(m) "sicut vos loqui deberem?" Schmidt. (n) "Etiam ego ut vos loquerer?" Cocceius; so Broughton. (o) "nectere deberem nexus contra vos verbis?" Schmidt. (p) "Adjungerem me super vos in sermonibus", Montanus, Bolducius; so Vatablus, Cocceius. (q) "Vobis enim aptum sermonem accommodarem", Tigarine version; so Codurcus. (r) "et caput meum quassarem super vobis", Cocceius; "movere deberem super vos caput meum?" Schmidt. (s) So Tigurine version and Bar Tzemach, , Hom. II. 17. v. 200.

heap up--rather, "marshal together (an army of) words against you."
shake . . . head--in mockery; it means nodding, rather than shaking; nodding is not with us, as in the East, a gesture of scorn (Isaiah 37:22; Jeremiah 18:16; Matthew 27:39).