Learn Polish: Bankomat jest przy banku. - The cash machine is by the bank.

The preposition przy (by, near, next to) always requires the Locative case. Therefore, the basic dictionary form bank changes to its Locative form, banku.

I thought the Locative case usually adds -ie to masculine nouns. Why does bank get an -u?

Most regular masculine nouns get the -ie ending in the Locative, but nouns ending in the letters k, g, or ch take the -u ending instead to make pronunciation easier. Because bank ends in a k, it becomes banku.

What is the difference between using przy banku and obok banku?

Both can translate to "next to" or "by", but przy suggests being very close to or right at the location. obok banku simply means "beside the bank," while przy banku feels more like the cash machine is right against the bank's building or on its immediate property.

If I want to say I am "at the bank" to do some banking, can I use przy banku?

No, przy banku specifically means physically near or outside the building. If you are inside doing business, you would use w banku (in the bank).