apt / liable / likely
Three words for what tends to happen.
Likely is the general word for probable: Rain is likely. Apt means having a tendency, often by nature or habit: apt to interrupt. Liable means exposed to something unwelcome, especially a risk or legal burden: liable to a fine, liable to flood. Do not use liable as a casual synonym for likely when no risk is in view.
Examples
Use The vote is likely to be close.
Use He is apt to forget names.
Use The cellar is liable to flood.
Avoid The vote is liable to be close.