is it correct to say: if you could see it then you would understand ( instead of
if you saw it then you would understand )
About your question, I can see on what your dubt is based: you've made a direct translation from Spanish to English. This is my tip for you: avoid literal or direct transaltions.
The sentences you've written are both right. However, it would be better for you and easier for other people to understand what you really are attempting to express if you adapted your own ideas to the English stereotypes of speech.
The second sentence you've written belongs to what's called "second conditional" in English. Notice that in Spanish it's not so different ("si lo vieras, lo entenderķas"). These, as much others, are similar syntactical structures between the two languages.
I hope it may have helped you!