The structural criterion

Structural invariability is an essential feature of PUs, though some of them possess it to a lesser degree than others. Structural invariability of PU finds expression in a number of restrictions.

Restriction in substitution

No word can be substituted for any meaningful component of a PU without destroying its sense.

The second type of restriction is the restriction in introducing any additional components into the structure of a PU.

The third type of structural restrictions in PU is grammatical invariability.

to find fault with somebody

e. g. The teacher always found f a u l t s with the boy (is not correct)