Right now he has nothing to send back. Once he gets the charges, the rules spell out the sequence. Trump is summonsed and ordered to answer, in writing. And so on.
There is lots of flexibility how the senate handles this and the grounds for decision. One of the grounds could be that the articles do not state an impeachable offense. I think Clinton made that argument, but the senate didn't vote on that.
Clinton actually did commit a crime. (I am firmly of the opinion that he did it to keep discussion of his sale of missile technology to China and nuclear technology to North Korea out of the news cycle; he was successful at keeping most of the American public in the dark.)
But the articles of shampeachment fail to identify a single crime. They are clear grandstanding by a Congress that is attempting to overthrow the government every member has sworn to uphold.
The silver lining here is that while the Congress was busy coaching non-witnesses and throwing hissy fits over the fact that they do not understand a President who does not share their hunger for power and ill-gotten wealth, the Senate has been busy confirming judge after judge without a single demagogic peep from the left.
Closed door meeting locking out the Dems. Allow Trump to challenge legality of Nadler’s unsworn witness testimony and all testimony of second hand information. Rule to exclude these Items from the impeachment process and move to dismiss. Done by lunch. Ready to vote on more Trump judicial picks.