What does the preliminary report on the Ethiopian B737 MAX accident show?
In the short summary, the preliminary report (33-page PDF available here) gives these details:


  Shortly after takeoff, the Angle of Attack sensor recorded value became erroneous and the left stick shaker activated and remained active until near the end of the flight. In addition, the airspeed and altitude values from the left air data system began deviating from the corresponding right side values. 


Among the few initial findings, the preliminary report has these:


  
  Shortly after liftoff, the value of the left angle of attack sensor deviated from the right one and reached 74.5 degrees while the right angle of attack sensor value was 15.3 degrees; then after, the [left] stick shaker activated and remained active until near the end of the flight.
  After the autopilot disengaged, the [Flight Data Recorder] recorded an automatic aircraft nose down (AND) trim command four times without pilot’s input. As a result, three motions of the stabilizer trim were recorded. The FDR data also indicated that the crew utilized the electric manual trim to counter the automatic AND input.
  


In the section giving details derived from the Flight Data Recorder and Cockpit Voice Recorder, the following items are included:


  
  At 05:40:00 shortly after the autopilot disengaged, the FDR recorded an automatic aircraft nose
  down (AND) activated for 9.0 seconds and pitch trim moved from 4.60 to 2.1 units. The climb was
  arrested and the aircraft descended slightly.
  At 05:40:03 Ground Proximity Warning System (GPWS) “DON’T SINK” alerts occurred.
  The column moved aft and a positive climb was re-established during the automatic AND motion.
  At 05:40:12, approximately three seconds after AND stabilizer motion ends, electric trim (from
  pilot activated switches on the yoke) in the Aircraft nose up (ANU) direction is recorded on the
  DFDR and the stabilizer moved in the ANU direction to 2.4 units.
  


A second similar sequence (automatic nose down trim, “DON’T SINK” from GPWS, and pilot-commanded electric nose up trim) then occurred from 05:40:20, to 05:40:28.

The diagram below, from a report appendix, shows aircraft pitch related data from the FDR from the first automatic nose-down to the end of the recording.  (Time advances from left to right.)




  
  At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer confirmed stab trim cut-out.
  At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance
  of AND automatic trim command occurred without any corresponding motion of the stabilizer,
  which is consistent with the stabilizer trim cutout switches were in the "cutout" position.
  From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved
  in the AND direction from 2.3 units to 2.1 units. During this time, aft force was applied to the
  control columns which remained aft of neutral position. The left indicated airspeed increased from
  approximately 305 kt to approximately 340 kt (VMO). The right indicated airspeed was
  approximately 20-25 kt higher than the left.
  At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied
  that the trim was not working and asked if he could try it manually. The Captain told him to try.
  At 05:41:54, the First-Officer replied that it is not working.
  At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,400 ft, two
  momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in
  the ANU direction from 2.1 units to 2.3 units.
  At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic
  trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in
  approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft
  column force was applied, but the nose down pitch continues, eventually reaching 40° nose down.
  The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.
  The left Indicated Airspeed increased, eventually reaching approximately 458 kts and the right
  Indicated Airspeed reached 500 kts at the end of the recording.
  


The "automatic aircraft nose down" events mentioned in the above can be ascribed to the MCAS (Maneuvering Characteristics Augmentation System), which Boeing only started informing pilots about in late November, 2018.