I had to go ahead and make a bunch of mistakes (or come close) so that you won't have to. These are tips on the local life that I hope will make your next trip to Bangkok as sweet as mango sticky rice.
Public transit can be free
The BRT (Bus Rapid Transit) is owned by a private individual, and after disagreeing with the government on fares, the owner decided to make the bus lines free of charge.
As a countermeasure, Google Maps will not display public transportation lines for most BRT-operated lines, and even claim bustling stops such as Sathorn are permanently closed.
Before I had the context, I would assume they were actually closed, and order a cab or get off the bus early and walk for 10 minutes under 35 degrees Celcius / 95 degrees Fahrenheit.
ATMs give you your money first, then your card
As it works the other way around for Turkey and the United States, I think my jet-lagged brain assumed I took out my card since I withdrew the money.
Well, an hour later, I couldn't find my debit card and connected the dots. Upon calling the bank, I learned that some ATMs push the card outside instead of keeping it in. I wasn't too amused.
You can commute with a motorcycle
The ride is a very thrilling experience, albeit dangerous, because you will be tailgating vehicles, zigzagging through lanes, and going at high speed without a helmet. Touching (holding the shoulders included) your driver is strictly prohibited.
However, this method is popular for a reason: The city has one of the most congested traffic I have ever seen and this is a way to scoot by. A lot of locals use it and are so used to it that I've seen a bunch of passengers playing games on their phone. You can set the price before the ride, or even call one from the rideshare & food delivery app, Grab.
You likely will have to eat without knives
Even when the food is not bite-sized, most Thai restaurants only bring a fork and spoon. I believe this is a tradition that goes back to King Rama IV's practices.
Wear socks when going to the temples
Since you will have to take your shoes off when going in, I recommend to have a pair of socks with you. Also, same with entering any other religious place, you have to cover up your shoulders, and any skirt/pants have to extend below the knees.
Line up
I have yet to use the subway, but for the bus, I can say that there is only one door for passengers. If you wait for the next one sitting on the bench, you will notice that plenty of people start lining up despite there being plenty of vacant benches.
They are all going to sit before you once you're inside. Cutting in line or fair game?
The bidets are cool
I have seen options where it comes as a lever attached to the toilet, which you can pull down to clean your front and pull up to clean your derriere.
Carry both cash and card
Some places accept one, some the other, and you usually won't know until you make it to the cash register. Thai people can also opt for a QR app for their payments. There has been some electoral promise to transition into digital wallets, but it received backlash as it was associated with control. So if you were thinking of using ApplePay, it's good to have some backup options.
Speaking of elections, don't speak of it!
Foreigners are not allowed to make any comments on politics or the monarch, and failure to comply is punishable by law.
Thai breakfast is not your typical breakfast
You would still have continental breakfast options given the huge expat population in the city, but Thai people usually eat fried chicken or pork for breakfast.
If you want to travel to another city and see a bargain in a touristy place, there's likely a catch!
I was very excited to see $60 roundtrip tickets to Phuket, with $30/night decent hotels. I assumed it was just the lower cost of living. Then I realized the monsoon season was coming, so it would rain the whole time.