What is a mental construction ?
Pali : Samkhara - ( mental ) formation, construction.
Remember that kamma ( action ) is re-defined by the Buddha as cetana ( intention or volition ). Volition builds habits and these habits are called mental constructions. Almost everything in a normal persons state of mind is habits, built up over many years. habits of feeling, habits of perception, habits of thinking, habits of craving, habits of attachment, habits of belief. The normal state of mind is a mass of habits. We learn to think, see and behave in certain ways. We become conditioned.
Much of this learnt behaviour is unavoidable and perfectly wholesome. But a portion of it is unwholesome. When Buddhism talks about eliminating these mental constructions it means the unwholesome ones - otherwise one would be reduced to the state of a new-born baby! But how can these mental constructions be eliminated ? The key is that these constructions result from mis-understandings of certain things. When these mis-understandings are corrected then the resultant constructions just dissappear.
This is why samkharas are said to be impermanent - it means capable of ceasing.
Best wishes, Vincent.