The combination of two particular linguistic units, or of two subclasses of units, may be constrained or even excluded. For instance, non-agentive verbs do not occur in commands (); and the Latin subordinative conjunction cum ‘because’ requires the subjunctive in its clause ().

A particular combination may be constrained or excluded for two different reasons:

1. for semantic reasons, i.e. because it does not make sense;
2. because of a structural constraint, i.e. because there is a rule of grammar to that effect.

Of the above examples, is of the first type. Once we have a definition of the concepts of agentivity and of the use of the imperative in a command, the restriction observed follows. In such cases, the combination of certain semantic units results in an incoherent thought. Often the combination of the structural categories in question may not in itself be ungrammatical.

For instance, the conditional construction seems acceptable because it does not involve the combination of the semantic units in question. Cases of semantic incompatibility are generally alike across languages because they do not depend on particular grammars. A rule excluding it is one of semantic coherence.

is of the second type. To the extent that the moods have meaning in Latin, there is no semantic reason why a causal clause should not be in the indicative. There are, in fact, other causal conjunctions such as quod ‘because’ that do take the indicative. Moreover, other languages that are like Latin in using subordinative conjunctions and distinguishing moods on the verb nevertheless do not have such a constraint on causal clauses introduced by a conjunction meaning ‘because’. This is, thus, due to a rule of Latin grammar. Such a rule is one of the symptoms of the advanced grammaticalization of the moods in Latin. Such a constraint is a structural or grammatical constraint.

Since grammaticalization is a matter of degree, there are borderline cases. For instance, the Latin moods do retain some meaning in some contexts. Then the subjunctive is the mood to mark propositions that are not presupposed by the speaker. For instance, the conjunction ut ‘as’ takes the indicative, as in a, where the way you give may be taken for granted.

The homonymous conjunction ut ‘in order that’ takes the subjunctive, as in b, where your giving is explicitly not presupposed by the speaker. The constraint saying that ut ‘in order that’ takes the subjunctive in its clause is, to that extent, semantically motivated. In such cases, there is usually cross-linguistic variation. Many languages side with Latin in requiring the subjunctive in (different-subject) purpose clauses. Other languages (among those that have moods, like German) admit the indicative. In such cases, we may speak of a (partly) semantically motivated structural constraint.