Q:

I thought they were/are 9us(size)

The thing that bothers me pretty much right now.
I know that I should use I thought they were ... in most cases.
The thing is, let me put it the straightest way I can.  
The other day I saw an offer of the shoes and in the description was mentioned only 9. I took for granted that it was 9 in US sizing, but it turned out that it was uk. Now the question, because the shoes are still for sale and they still exist - in that case, should I say "I thought they are 9us", or no matter what should I go with "I thought they were 9us". 

A:

You thought they were size nine (US) at the time you ordered them (in the past). You now (in the present) know they are size nine (UK). 

I thought they were US size nine but now I know they are UK size nine. 
When I met my husband I thought he was a nice guy but now I know he's a total jerk. 

Previous assumption, present knowledge.

