Question

how would you learn the value of leaf and leaflet​

Answer 1

When they have multiple leaflets they're called compound leaves. You can tell the difference between a compound leaf's leaflet and a simple leaf by checking where the leaf attaches to the stem. If there is no axillary bud, then it's a leaflet and not a leaf.

A leaflet (occasionally called foliole) in botany is a leaf-like part of a compound leaf. Though it resembles an entire leaf, a leaflet is not borne on a main plant stem or branch, as a leaf is, but rather on a petiole or a branch of the leaf.

A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Leaves are collectively referred to as foliage, as in "autumn foliage".