What is the correct grammar for If you were to go to the store?

"If you were to go to the store" is a clause that introduces a cause and effect relationship. Note that it is a hypothetical condition that requires the subjunctive mood of the verb "to be". The sentence could be finished with a condition that will occur as a result:

If you were to go to the store you would see that the turnips are on sale.

The use of "then" to join the two is optional.