Reaction of acetaldehyde with formaldehyde?

Good Midnight In The Midnight,

Task:

" Draw the reaction products and state the name and/or class of substance. Take into account the occurrence of stereoisomers, if applicable."

Solution:

Problem:

I don't quite understand the solution. I know that the one on the left is acetaldehyde and the one on the right is formaldehyde. Now there's a negative charge on the left because there's nothing attached to it. So we have a nucleophile here. The nucleophile is now attached to the point marked in red because there's a positive charge there, as the oxygen atom attracts the electrons from the carbon. That's not right.

Now I need to attach the nucleophile to the red dot. It looks like this:

So, now we have the problem that the carbon above has five bonds. I would have removed one of the two H bonds, but our professor now moves the upper double bond of the oxygen atom one upward. I don't quite understand this. Wouldn't it be easier to simply remove one H and split it off somehow? Also, in the solution on the right, there's suddenly an OH. How did the H get there? Did he simply move the double bond of the oxygen upward so that there's a free electron pair, and then, as if by magic, fit one of the H atoms in there. So, like this:

=>

But then there would be one H too many.

I'm sorry, but I really don't understand how exactly the substitution was carried out here. The aldehyde is attached to the carbon atom of the formaldehyde. But how one arrives at the solution is a mystery to me.

LG, Jensek81