on the question of how it helps that Celtic languages have (optional) do-periphrasis:
The earlier argument was that English is the only Germanic language that has (obligatory) do-periphrasis, and the question was where this came from. It has been discussed by a number of scholars, whether Celtic, particularly Welsh, influence could have played a role. This argument was that periphrastic 'do' was not there in the other Germanic languages, but it was there in Celtic. That it became grammaticalised as an obligatory marker in Early Modern English is not really a problem for the question of how it came into being in the first place. 
And of course English differs from the less westerly Germanic languages in that it had multiple contacts with speakers of other languages coming into England, whereas German or the Scandinavian languages have had rather less contacts. But if we say that, right, the presence of do-periphrasis therefore may be due to contact with Celtic, what do we do with the dialect features (which were not really on the radar of many scholars coming from an English language background). And two problems have been mentioned:

1. do-periphrasis is not grammaticalised in the central Germanic dialects/languages
Perhaps those languages have been under less pressure to restructure as they were not digesting the input of ,first, speakers of Celtic, then Norse and then Norman French. Still, a lag of a couple of centuries remains between putative Celtic influence and final grammaticalisation. A question here is, how long a potential lag are we prepared to accept till final grammaticalisation?
A very interesting point is the question of Yiddish, which Dennis brought up. That of course would have had multiple influences depending on  where the speakers came from, and live(d). Does Yiddish have a more analytic verbal system than other Germanic languages or dialects?
As to whether 'do' is really used for emphatic purposes in Germanic dialects: the impression I got in the Rhine-area was that it can be, of course, as it can in English, but in many cases it is not: it is typically unstressed and the contexts are typically unemphatic.

2. German dialects had no contact with Celtic
I heard an argument somewhere (and I do not remember who put it forward, unfortunately, or when) that it might even be an indicator of a Celtic substrate that some continental Germanic languages in former Celtic settlement areas have do-periphrasis. I remain somewhat sceptical on that one and I could think of a number of alternative explanations for the presence of do-periphrasis in the dialects, some of which have been mentioned in the discussion.

Elliot's comment that one would expect Old English to take on the gerund, rather than the infinitive, if Celtic plays a role is an interesting observation. But it seems that early English did not favour gerunds as complements of content verbs. I have been looking at verb and verbal noun/nominalisation combinations both in Old English and Old Irish for a while now, and there are quite a few examples of optional do-periphrasis, both in 9th/10th century Irish and English - though not as many as in Middle Welsh. The early English material largely consists of 'do' plus nominalisations or nouns connected to a verb e.g. cigan ‘to invoke X’ cignesse don ‘to give an invocation', or bysne don, bysnian ‘to set an example'. In contrast to verbal nouns in Old Irish/ Celtic, gerunds do not seem to have fitted into the slot of 'verbal complement' in Old English very well. 


> Neil asked my question for me. Also, if English was going to develop
> periphrastic 'do' on a Celtic model, one may have expected a wider use of
> gerunds (i.e. verbal nouns) in this construction and not bare infinitive forms
> (which are much much less nominal than gerunds).
> So, instead of  'Do you see him?'
> One might have expected 'Do you seeing of him?' 
> -Elliott
> ----- Original Message ----
> From: Neil McLeod <[log in to unmask]>
> To: [log in to unmask]
> Sent: Friday, March 20, 2009 8:03:00 PM
> Subject: Re: [OLD-IRISH-L] dogni+VN (McWhorter)
> Dennis King wrote:
> > Kai-Oliver Geisler wrote:
> > 
> >> "Ich tu' mal das Fenster zumachen"
> >> or: "Tust Du heute noch die Wäsche abnehmen?"
> > 
> > McWhorter writes that "in many colloquial dialects of Germanic
> languages, one can use 'do' in a way kind of like English's meaningless 'do'."
> > 
> > He claims, however, that compared to English, this usage "is something
> quite different", as follows:
> > 
> > 1.  "For one, German's version is optional.  One might say 'Er tut das
> screiben,' but the simple 'Er schreibt das' is also alive and well, and in
> fact, much more usual."
> > 
> > 2.  "Then, most importantly, meaningless 'do' is meaningless, but
> German's 'do' is meaningful.  It is used when you want to emphasize some part of
> the sentence."  He goes on to illustrate this.  Whether or not this
> argument is accurate is up to you native speakers of German dialects to determine.
> > 
> > 3.  "History of English specialists seem to suppose that it's just that
> English merely drifted one step beyond German's 'do' -- making it required
> instead of optional.  But if that were so natural, so same-old same-old,
> then surely it would have happened in some other Germanic language sometime
> [...] surely some small dialect of something somewhere -- some villagers in
> the northern reaches of Sweden, some farmers down in some Dutch dell, some
> Yiddish speakers in a shtetl -- somebody, somewhere would have come up with
> their own meaningless 'do' just by virtue of shitte happening. But they
> haven't." But (as to 1 and 3 at least), if it is also optional in Celtic, how
> does that help? 

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