The movie was getting boring so I left.
The movie was getting boring that's why I left.
In #2, you have incorrectly joined two sentences together. You need to say eg The movie was getting boring so that's why I left. eg The movie was getting boring and that's why I left.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
In #2, you have incorrectly joined two sentences together. You need to say
eg The movie was getting boring so that's why I left.
eg The movie was getting boring and that's why I left.
There's no real difference in meaning.
Clive
What if OP said "The movie was getting boring. So I left."
Or "The movie was getting boring. So, I left."
Are they OK?