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My father was a jack of all trades in those days, as were all village school headmasters: teaching arithmetic
to the top class in such time as he had free.

In my book, such + noun + as can only be used in negative sentences.
Can you tell me, please, which comparative structure does the PP underlined contain.
I do not understand the structure of the PP, and I do not know about any such construction as this:
assertive nominal non-finite subordinate clause + such + noun + as + VP.

Thanks in advance to my trusted forum comrades.

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InchoateknowledgeMy father was a jack of all trades in those days, as were all village school headmasters: teaching arithmetic
to the top class in such time as he had free.

In my book, such + noun + as can only be used in negative sentences.
Can you tell me, please, which comparative structure does the PP underlined contain.
I do not understand the structure of the PP, and I do not know about any such construction as this:
assertive nominal non-finite subordinate clause + such + noun + as + VP.

Thanks in advance to my trusted forum comrades.

This is what I would call "too much sentence". If they were jacks of all trades, why mention only one? I suggest something like: ".....headmasters. He taught...........free and [something else that he did]", making shorter sentences.
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Inchoateknowledge "in such time as he had free"
can I compare a noun with a verb phrase like this?

You're making the wrong assumption here.
This isn't a comparative structure.

as=that, which
here
(check a dictionary, it's one of the meanings)

This means:
in the time which/that he had available
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Comments  
thanks Philip

"in such time as he had free"
can I compare a noun with a verb phrase like this?
in such time as he had free = in whatever time (that) he had free = in whatever free time (that) he had

This is not a very common construction! It focuses on scarcity.

Mr. Thomas had insulted almost everyone. Most of his friends turned their backs on him. Now he had to content himself with such friends as he had left.*
Gregory was nearly broke. He had to make do with such financial resources as he had available until his next paycheck.
Sue Ann had a terrible argument with her sister. Eventually, when they began to speak to one another again, it was only with such civility as they could muster.


*Clarification: Not that he had left (abandoned) his friends! Rather, these were the only friends that remained (were left).

CJ
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 Marius Hancu's reply was promoted to an answer.
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as


Function: pronoun

1 a : THAT , WHO , WHICH -- used to introduce an adjectival clause and having same or such as antecedent <their children should grow up in the same intellectual culture as they have enjoyed -- G.B.Jeffery> <tears such as angels weep burst forth -- John Milton> b now dialect: THAT , WHO , WHICH -- used to introduce an adjectival clause and having a noun or pronoun as antecedent <a lot of things happened ... as never ought to -- Richard Llewellyn> <was going to tell the gospel to them as had ears -- R.P.Warren>

2 : a fact that : THAT <he is a foreigner, as is evident from his accent> <I have used thee, filth as thou art, with humane care -- Shakespeare>


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Incho:
I think you should read less grammar and more novels/fiction. Seriously.
Perhaps you're already doing that.
Just my 2 cents.