Joonas's answer is entirely correct, but to add one note:
Sometimes it is possible to, apparently, combine prepositions in Latin. For example, a punishment ex post factō comes from (ex) a law created (factō) after (post) the action itself happened, with two prepositions (ex and post) in a row.
However, when this happens, it's usually (always?) because one ...
Ante a priori is gibberish.
Go with ante priorem instead.
"The former" in Latin is prior.
This word can be inflected to a number of different forms depending on the situation.
The relevant forms or cases here are the accusative priorem and ablative priori (also priore).
The opposite word posterior has the same endings for the same forms.
The preposition a (...