Grammar Help

This is something my English teacher told me: A semicolon can be used to join two short sentences that would have otherwise been separated by a period. Perhaps you could use this as a benchmark to decide when to use a semicolon.

Yea that sounds right. You should only use a semi-colon if you know what you're doing. The only time I use it [and was permitted to in ap english otherwise points would be taken off] is when using it with words such as

however
nevertheless
rather
furthermore

etc..

So the sentence structure is either

A) John went to the store. However, it was closed

B) John went to the store; however, it was closed.