Comma and 'that' omissions in the sentence

Question

I have four ways to write my sentence:

(comma separated)
The implementation, we have made before, may ...
The implementation, that we have made before, may ...

(without commas)
The implementation we have made before may ...
The implementation that we have made before may ...

Which one is correct and sounds more natural?
Which rules are applied here?
Should I use that here?

In my native language, the part which provides some additional information is always highlighted by commas. It looks logical and more readable for me.

Any assistance would be appreciated. I'll be grateful for a detailed explanation with related references.

Answer 1

Of the four structures given,

1. *The implementation, we have made before, may ...
2. *The implementation, that we have made before, may ...
3. The implementation we have made before may ...
4. The implementation that we have made before may ...

(1-2) are both ungrammatical (whence the asterisks) because commas make a normal
relative clause into a non-restrictive (parenthetical, supplementary) relative clause,
which

• has different syntax from restrictive relative clauses,
• requires a relative wh-pronoun (which, who, etc),
  which
  • can't ever be deleted like (1), and
  • can't ever use that like (2).

(3-4), on the other hand, are both grammatical because they are restrictive (normal, integrated) relative clauses, and thus are not subject to those restrictions. Restrictive relative clauses

• are not set off with comma intonations at either end

• are allowed to use either a wh-pronoun or that, regardless of the antecedent

  • The man that came to dinner broke his leg.
  • The man who came to dinner broke his leg.
  • The leg that he broke healed rapidly.
  • The leg which he broke healed rapidly.

• are not required to have a relative pronoun at all, except as subject

  • *The man came to dinner broke his leg.
  • The leg he broke healed rapidly.

Answer 2

It depends on what you're trying to say.

Let's start with:

The implementation we have made before may become a cause of code duplication.

Does "the implementation" refer to the current implementation, or are the two implementations identical?

If so, then it should be:

The implementation, which we have made before, may become a cause of code duplication.

If "the implementation" refers to a previous implementation, then it should be:

The implementation that we made before may become a cause of code duplication.

People often suggest rather strongly to omit "that" when used as a subordinating conjunction; I prefer to leave it for the sake of clarity. There are instances in which omitting it introduces ambiguity.

As for your question about pauses: pause at the commas, if they are used.

Peripherally, you may want to remove the "have", as it implies an ongoing process--as if you are continuing to build the original implementation.

Here's an article with more detail.