https://core.ac.uk/download/pdf/33527822.pdf - p.146

The Verb Inflexions86
In the 2nd P. Sing, Pres. the "t" is sometimes omitted, from the syncopated form' when the verb has the terminal - sound "t" or when the following word has the initial sound "th" (thou affects = thou affect'st; mean(e)s thou? =- mean'st: thou?), or when the combination of words in a sentence produces groups of consonants that would be inadmissible in a single word: thou mistakes me, thou makes me call ("t" is suppressed as in modern Christmas, chestnut); it is rarely omitted apart from this. Sometimes questions of syntax are involved (see the section on Congruence), but usually the reason for the omission is simply the need to facilitate pronunciation* The same is true of the syncopated form of the 2nd P. Sing. Pret. (lik'st = lik'dst,; look ! st = look'dst; unrip f st = unripf dst).
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The forms with "d" are difficult to pronounce and cannot have been common in normal speech.The use of forms like are, will, shall, should, made with the 2nd P. Sing, usually has a different explanation. Tk2 singular and plural forms are connected here. In scne casesiphonemic considerations may also be involved (would thou? for wouldst thou?; shall thou? -Tor shalt. thou?).

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