If we think of the Russia meddling/Trump collusion investigation as a wheel, there are about half a dozen spokes holding its structure: whether the Trump campaign colluded, whether they did it wittingly or unwittingly, whether the president or his team tried to obstruct justice afterward, and what, if any, financial ties members of Trump's campaign have with Russia.

Donald Trump Jr. is connected to many of those spokes.

That's why his testimony to Congress could be so crucial to the various federal investigations into Russia meddling in the presidential election and whether the Trump campaign helped. On Thursday, the Senate Judiciary Committee is questioning Trump Jr. It is his first time to testify before Congress about all this.

The testimony will take place behind closed doors, but The Post's Karoun Demirjian reports it's expected to be a comprehensive meeting with committee staff and some lawmakers popping in and out. It's unclear if the public will get a full rundown of what was said, but follow Demirjian for up-to-the-minute news of the meeting.

In the meantime, here are four critical questions about the investigations that Trump Jr. can help shed light on.

1. Why did Trump's campaign meet with Russians in the first place, and what were the circumstances?

This gets directly to the heart of the parallel FBI and congressional investigations: Did Trump campaign officials intend to collude with Russia to help Trump win? Proving this would be the crux of any legal case against the Trump team.

In June 2016, Trump Jr. got an email from a friend telling him that a Russian lawyer, who has connections to the Kremlin, wanted to meet with him to share dirt on Hillary Clinton. Not only that, Trump Jr. was told that this was part of a Russian government play to help his father win an election.

He took the meeting, responding to the Clinton news: “If it's what you say I love it.”

(Image of some of the emails Trump Jr. released in July about the meeting)

Legal experts say that matches the textbook-legal definition of conspiring to obtain information from a foreign adversary, or collusion.

“It's a shocking admission of a criminal conspiracy,” said Jens David Ohlin, associate dean of Cornell Law School, in a statement shared with The Washington Post when Trump Jr. released these emails.

So, obvious question here: Why did Trump Jr. take the meeting in the first place? And why did Jared Kushner, a senior adviser to Trump and his son-in-law, as well as Paul Manafort, the former campaign chairman for Trump, come along? If it was to get dirt on Clinton from Russia, well, that'll be big news for the investigation.

2. What kind of relationship did members of the Trump campaign have with any Russians in that Trump Tower meeting, before, during and after it?

The Russian lawyer promising dirt on Clinton, Natalia Veselnitskaya, wasn't the only shadowy character in the meeting. Slowly, details leaked out that a Russian-American lobbyist known for wily political tactics was there, as was the vice president of a Russian real estate company, who two decades ago was the subject of a congressional inquiry into Russian money laundering in U.S. banks. All in all, there were eight people in the meeting.

In other words, this doesn't appear to be your average meet-and-greet meeting to build relationships between a presidential campaign and a foreign government.

Jeffrey Jacobovitz, a white-collar lawyer who represented members of the Clinton White House during scandals, said investigators will probably want to know how all these people are connected. And one way to get at that is to talk about how the meeting was set up in the first place.

“There were a lot of parties at the meeting,” he said. “Was it out of the blue? Or was it a prior relationship?”

3. How much involvement did the president have with this meeting?

Donald Trump Jr. and then-candidate Trump in February 2016. (Jim Bourg/Reuters)

Here's where investigators are likely to delve into the obstruction of justice spoke on the Russia/collusion wheel. After Trump fired his FBI director, special counsel Robert S. Mueller III is investigating whether the president obstructed the FBI's investigation.

Now, we know that three very senior members of Trump's campaign held a meeting about dirt on Clinton. A year later, when the New York Times reported on this meeting, the president himself dictated a misleading statement on why the meeting was held in the first place.

As The Washington Post reported in July: “Trump personally dictated a statement in which Trump Jr. said that he and the Russian lawyer had 'primarily discussed a program about the adoption of Russian children.'”

After that, Trump Jr.'s explanation for the meeting changed three times, ultimately settling on: He took the meeting to receive “very high level and sensitive information” that was “part of Russia and its government's support for Mr. Trump.”

That's, uh, very different from adoptions.

Investigators will almost certainly want to corroborate with Trump Jr. his father's involvement in crafting that original, misleading statement. And it's fair to ask: If the president dictated this, what else was he involved in with regard to this meeting?

4. What came out of the meeting?

Another spoke on the collusion wheel is any financial ties connecting Russia and members of the Trump campaign.

Trump Jr.'s main defense for taking the meeting was that nothing came out of it. It was a bust that was over in 15 minutes, he said. But details that have emerged afterward suggest that may not be the case.

And as The Fix's Aaron Blake points out, Trump's side used the fact that Manafort was on his phone in the meeting to illustrate how boring, useless and unimportant the meeting was.

But then we learned Manafort was taking notes — on his smartphone. Some of the notes, NBC reported, were cryptically about political campaign contributions.

So investigators will want to know: How useful was this meeting? And did you continue to follow up with these folks? Was there any money exchanged? They'll probably ask for Trump Jr. to provide any emails or other documents related to potential follow ups.