What do you wish you knew about your spouse before getting married?

Men think women will never change and they do

Women think men can change but they don't

That basically summarizes my experience (including lots of negatives re me not changing). I think managing those expectations is much more important than knowing some particular thing, which in any event would probably map to my aphorism.

(I generally reject gender stereotypes, feel free to swap the gender anywhere to match your situation, the point is about expectations for the future)

There's going to be culture shocks. For example, what I think is a good job or career direction isn't the same as what she thinks, and vice versa. For some, a job with a pension means success.

Another might be well, small things. When my mom puts a plate of cookies on the dinner table, it's for everyone. She gets offended if the cookies are still there in the morning. When my wife's family puts a plate of cookies on the table, it belongs to someone. You ask before taking any, because it could be reserved for a specific person. This leads to a lot of arguments, assumptions about how selfish other people are, etc.

There's also skills. Some people are not financially fluent - they may buy a $2000 thing and pay $50 over 5 years or so.

Some are not very scientifically fluent - I have relatives who will wash the dishes in any clear liquid, including wastewater, because they assume that if it's clear, it clean. Or some might assume that the lights use up the most electricity when it's bright, so they go for the dimmer ones with higher wattage.

Neatness too. I'm glad my wife is a minimalist like me. Some people love having lots of stuff; it makes them feel wealthy and inspired.

If she can code in Lisp ;)