preposing - "and build upon that, but build they have": Should that 2nd "build" be "built"?

From the book Thinking in Java:

The .NET platform is roughly the same as the Java Virtual Machine (JVM; the software platform on which Java programs execute) and Java libraries, and C# bears unmistakable similarities to Java. This is certainly the best work that Microsoft has done in the arena of programming languages and programming environments. Of course, they had the considerable advantage of being able to see what worked well and what didn't work so well in Java, and build upon that, but build they have.

I'm not sure how to properly understand that part. First of all, is that even correct? I mean build, shouldn't it be built?

1. 
   
   

   and build upon that, but build they have. -- [original]

   
   


2. 
   

   and build upon that, but built they have.

   
   

Is version #1 with build, as in the original, correct? Or is version #2 with built correct? Or are both correct?

Clear things up for me, please.

Answer

1. and build upon that, but build they have. -- [original]


2. and build upon that, but built they have.

In general, both versions are acceptable in today's standard English.

But in this specific example, there would often be a preference for version #1, which is the original version that had used "build":

• 
  
  
  
  
  1. and build upon that, but build they have. -- [original]
  
  
  
  

Version #1 would often be preferred because the second "build" would then match the first "build" which was used in the previous clause, and that would give a rhetoric effect which would often be desirable by the speaker.

Note: A related answer post to this grammar issue is: “Wrote it I did” Is this grammatical?

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LONG VERSION

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Your example uses complement preposing in the second clause, where the preposed element is a verb phrase (VP). Usually the second clause will involve an auxiliary verb when the preposed element is a VP.

Here are some typical examples. The 2002 CGEL, page 1376:

• 
  

  [11.i ] I've promised to help them [ and help them I will ].

  
  


• 
  

  [11.ii ] It's odd that Diane should have said that, if [ say it she did ].

  
  

The preposed VP in [11.i ] is "help them", and in [11.ii ] it is "say it". Notice that the nucleus of the second clause in both examples end with an auxiliary: "will" for [11.i ], and "did" for [11.ii ].

Here are their corresponding versions that don't have the preposing:

• A.i. I've promised to help them and I will help them.


• A.ii. It's odd that Diane should have said that, if [ she said it ] / [ she did say it ].

But when the auxiliary verb is the perfect "have" and the preposed element is its complement, then both the past-participle form and the plain form of the verb are acceptable.

The 2002 CGEL page 1381:

Inflection with perfect have

A special issue arises when the preposed element is a complement of perfect have. Compare:

[25]

• 
  

  i. He said he wouldn't tell them, [ but tell/told them he has ].

  
  


• 
  

  ii. He denies he has told them, [ but tell/told them he has ].

  
  
  

Although have normally takes a past participle, it is the plain form of the verb that is preferred in [i ]. The past participle is preferred in [ii ], where it has been used in the preceding clause, but even here the plain form tell is acceptable.

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NOTE: The 2002 CGEL is the 2002 reference grammar by Huddleston and Pullum (et al.), The Cambridge Grammar of the English Language.