The weather changed from sunny to rainy in seconds.
Suddenly / Immediately
You realised you were in the wrong classroom.
Eventually / Suddenly
You studied a little bit each day and improved your English.
Gradually
The door opened without warning
Suddenly
You knew who was calling before you looked at your phone.
Obviously
You tried the cake and it was very, very delicious.
Absolutely (→ absolutely delicious)
You didn’t check the bus times, so you missed it.
Obviously / Unfortunately
You dropped your ice cream, and there isn’t another one.
Unfortunately
You found some money in your pocket.
Fortunately
You didn’t understand what the teacher meant at first, but after rereading the question, you got it.
Eventually
The noise in the room became quieter and quieter.
Gradually
You saw two people whispering and pointing at you.
Obviously