(CNN) More than three weeks after President Donald Trump was impeached by the House, his impeachment trial has not begun in Senate.

These are the articles the House voted on in December for abuse of power and obstruction of Congress, after an investigation into Trump's pressure on Ukraine to investigate Democrats before the 2020 election, following a whistleblower report filed in August. Here is the final report on what Democrats found.

Read on for a refresher on how we got to a place where Trump has been impeached but not yet tried:

Why hasn't the trial begun?

Impeachment is two-step process in the House. First, members of Congress voted to impeach Trump on two articles -- abuse of power and obstructing Congress -- after holding weeks of hearings. But then they were supposed to conduct a third vote, to appoint managers who would present the articles in the Senate and basically serve as prosecutors. They have not yet taken that last vote. Read more here about impeachment managers.

What's the reason for not appointing impeachment managers and sending the case to the Senate?

Pelosi, who as House speaker controls what gets onto the House floor, has said she decided to hold the articles to make sure the trial in the Senate would be more fair.

The irony here is that Democrats moved impeachment through the House very quickly, without waiting for courts to force testimony from key witnesses -- in part to avoid a pileup in January (i.e. now) ahead of the upcoming Iowa caucuses. Republicans were complaining everything was going too fast. But now Democrats are trying to slow things down, and Republicans want to simply end the whole thing and put away the case against the President as soon as they can in a Senate trial.

Why didn't Pelosi think the trial would be fair?

Several senators, including Senate Majority Leader Mitch McConnell, said they were not exactly impartial jurors in the matter . And McConnell said back in December he had met with White House attorneys to coordinate on the trial.

Is it required for senators to be impartial? This is, after all, a political exercise.

Actually, yes. Article II of the Constitution leaves a lot of the details of an impeachment trial up to the Senate, but it does make clear that no impeached president can be removed unless two-thirds of senators -- usually 67 -- agree and also that when sitting for an impeachment trial, "they shall be on Oath or Affirmation."

What's the oath senators will take?

The oath, which senators must take before trying an impeachment case, is spelled out in Rule XXV of the Senate's rules on impeachment and specifically mentions impartiality:

"I solemnly swear (or affirm, as the case may be) that in all things appertaining to the trial of the impeachment of (the person on trial), now pending, I will do impartial justice according to the Constitution and laws; So help me God."

There is some wiggle room here, maybe, since McConnell says he won't be "an impartial juror," not that he won't be doing "impartial justice according to the Constitution," which he will presumably swear to do.

What has McConnell said about Pelosi's demand?

He has repeatedly demanded that Pelosi send over the articles of impeachment and warned she has no leverage to affect the conduct of the Senate trial.

Could he have started the trial before she sends the articles?

Not really. He would have had to change Senate rules, which would require a supermajority of senators (and help from Democrats). So McConnell and Pelosi are in a standoff.

What happened this past week?

The week began with no clear resolution. Pelosi said Wednesday she would send the articles when she is ready and that will probably be "soon." McConnell said she has no leverage over the Senate. He also has signed onto a proposal by Republican Sen. Josh Hawley of Missouri to change Senate rules to dismiss the impeachment articles if they aren't submitted within 25 days of being passed.

But, again, that would require a supermajority and Democrats likely won't sign on. By Friday, Pelosi relented and made clear she will transmit the letters next week. She argued that in the intervening three weeks, more was learned about the White House effort to withhold security aid from Ukraine

Why didn't McConnell just change Senate rules?

Changing Senate rules and ending the filibuster -- more commonly referred to as a "nuclear option" -- is something McConnell appears to have no interest in doing.

What could McConnell do to make Pelosi believe the trial will be fair?

The top Democrat in the Senate, Chuck Schumer, has asked that four witnesses be allowed at the trial. Pelosi has endorsed this move. One of those requested witnesses, former national security adviser John Bolton , has said he'll testify if subpoenaed.

Will McConnell allow witnesses at the Senate trial?

He does not want to even though Trump has said as recently as Thursday he'd like to see both the whistleblower and Joe Biden's son Hunter Biden testify as well. (It was Hunter Biden's appointment to the board of the Ukrainian company Burisma that Trump said should be investigated by the Ukrainians. There is no evidence of wrongdoing by either Biden.) On Friday, Trump suggested he would block Bolton's testimony as a matter of executive privilege.

Why was Bill Clinton impeached?) The two previous presidential impeachment trials -- for Bill Clinton in 1999 and Andrew Johnson in 1868 -- both featured witnesses. The Clinton witnesses -- three of them, including Monica Lewinsky -- were interviewed behind closed doors and then video of the testimony was shown. For Johnson, the witness interviews were conducted in the Senate. (Read more on those cases here: Why was Andrew Johnson impeached?

Why doesn't McConnell want witnesses? Do any Republicans want witnesses?

He wants to get this process over with and move on. He has so far punted on the issue and Republicans are following him. They have a majority in the Senate and he says he has the votes to make the trial a three-stage process . House managers would present their case and Trump's defenders, we don't yet know who the President will pick, will rebut them. And then, after those arguments, senators could ask questions through Chief Justice John Roberts, who will preside.

The third phase, according to McConnell, would be for senators to vote on whether to call witnesses. Several Republicans, including people like Maine's Susan Collins and Utah's Mitt Romney have expressed interest in hearing from witnesses. But they won't have to make a final decision for some time under this format. McConnell argues this is the model used for the Clinton impeachment. Pelosi disagrees.

How would senators go about calling witnesses?

Four Republicans would have to side with Democrats to give them a majority and summon each witness. Those will eventually be very interesting votes. But we'll have to wait. It's not clear how long.

What happens next?

The bottom line is that Trump is impeached, but his trial won't start until after Pelosi send the articles to the Senate. That will start a whole new drama in the Senate.