For what it's worth, this is what I currently see:
We already know that Trump ordered the withholding of aid to Ukraine a short time before the phone conversation with Zelensky. He has openly said so several times. His current explanation as to why he did so is for two reasons.
1) He felt that it was unfair that only the United States should put up the money, so he wanted other countries, particularly in Europe to put up money also. Apparently Trump didn't know nor did I, that Europe and it's countries already pay more than we do.
2) He wanted to make sure Ukraine was honest because Trump didn't want to give money to a corrupt country. This reason is why Trump asked the rhetorical question of why should we give money to a corrupt country?
We already know that Trump wanted Zelensky to look into the crowdstrike/server theory wherein Ukraine and the Democrats conspired to hack the DNC and then say it was Russia. We know this from his telephone conversation with Zelensky. That, by itself was asking for a personal favor that would be unethical since it would benefit him politically by raising the perception that the Democrats plotted the whole Russia interference investigation so as to smear him. Hence Trump calls it a hoax. Trump also mentioned that Zelensky should investigate Biden/Burisma in the phone call.
So enters in the whistleblower. The whistleblower not only gives a report that Trump is asking for personal political favors from the new Ukrainian President that will help Trump win the 2020 elections, but he also is pressuring Zelensky to carry out those favors. The Whitehouse will not release any documents concerning this, nor will they allow staff to testify. But testimony given so far by those who defied the Whitehouse or are not currently employed there any longer, has shown an effort by some official U.S. diplomats and also unofficial operatives working to get these investigations announced publicly by Ukraine in exchange for a white house meeting that Zelensky needs to show U.S. support of Ukraine against the agression of Russia.