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
UseThe vote is likely to be close.
UseHe is apt to forget names.
UseThe cellar is liable to flood.
AvoidThe vote is liable to be close.