What is the proper past tense for the word by?

The word "by" is not a verb (action word), it's a preposition (it describessomething's position in relation to something else); because of this, it has no past tense. Only verbs have past tenses.

Example: You could say "I go to the store by my house."

but you could also say "I went to the store by my house."

The first sentence is in the present tense, while the second sentence is in the past tense. In both sentences, the word "by" doesn't change, because it is used to describe the store's location in relation to the house. The word "by" performs no action on "I".

In short, "by" has no past tense, because it performs no action in the sentence.