Apt, likely and liable are often interchangeable when followed by to and the infinitive, but clear differences may be descried.
Apt implies an inherent or habitual tendency (as an inclination, bent, or predisposition); it refers to the past and the present as much as if not more than to the future; it applies commonly to persons, but may apply to things that show a tendency or drift.
- you are just a little apt to let yourself be a slave to that house of yours
—Bennett - the upper circles . . . are apt to favor a pronunciation derived . . . from that which prevailed in England
—Grandgent - long poems are always apt to drop in places into what is only not called prose because it is metrical
—Alexander
Likely stresses probability; it refers in time to the future and therefore has its place in predictions.
- he is likely to succeed
- the wedding is likely to cost her parents more than they can afford
- it is likely to rain today
- “How now! is Jack Cade slain?” “No, my lord, nor likely to be slain; for they have won the bridge”
—Shak.
Liable (see also LIABLE, RESPONSIBLE) implies exposure to a risk or danger; it suggests a chance rather than a probability and is therefore often used in warnings, in cautions, or in the expression of fears Children who play in the street are liable to be injured by automobiles).
- drivers must remember that cars are liable to skid on wet roads
- anyone who disobeys this rule is liable to be punished severely
Thus, a person who is apt to lose his head under stress knows that he is likely to fail when he is put to a test and therefore forms the habit of avoiding situations in which he is liable to encounter difficulties.