The characteristic property of to-infinitives (indeed of most infinitives) is that they have a silent subject: an infinitive cannot assign case to its subject. Hence the subject of the infinitive cannot be lexicalised. As a result, there are two options for infinitival subjects:

1. Control: The subject of a to-infinitive may not be lexicalised so it remains implicit. It gets its interpretation as a logophor, usually from an argument of the selecting auxiliary. This type of infinitive is called a control infinitive within generative grammar.
2. Raising: The subject of the infinitive must be lexicalised. Since the infinitive cannot provide a position of lexicalisation, the subject is lexicalised in an argument position of the auxiliary selecting the to-infinitive. This type of infinitive is called a raising infinitive within generative grammar.

The following types of to-infinitives will be distinguished, depending on the following options:

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• Does the to-infinitive require lexicalisation (raising) or control of its subject?
• Does the to-infinitive require lexicalisation (raising) of its object?
• Is there clause union, that is, union of the middle fields of auxiliary and main verb?

This yields the following classification of to-infinitival constructions.

• Those involving subject raising, that is, auxiliaries of which an argument positions hosts the subject argument of the infinitive. There are no idiomatic restrictions on the infinitive.
• Those involving object raising, that is, auxiliaries of which an argument position hosts the object argument of the infinitive. In such cases, the subject is always controlled.
• Those involving control of the subject of the infinitive by an argument of the auxiliary, usually the subject, but with clause union and idiomatic restrictions.
• Controlled but without clause union.
• Controlled infinitives which neither lexicalise nor assign case to their object. As a result, only reduced infinitives can enter this construction.

The complexity of the behaviour of to-infinitives necessitates this classification. Note that according to this classification an infinitive always has a subject, which is either lexicalised elsewhere or controlled. In contrast, objects may not be controlled, but they may be lexicalised elsewhere. It will be explained in more detail in the topics below:

• 4.1. Raising auxiliaries selecting to-infinitives
• 4.2. Object oriented to-infinitives with clause union
• 4.3. Subject oriented to-infinitives with clause union
• 4.4. Subject oriented to-infinitives without clause union
• 4.5. Reduced and full to-infinitives
• 4.6. Noun incorporation