The infinitive is in part a verb, in part a substantive.
a. Many substantives are closely related to verbs, but not all verbs can form substantives. All verbs can, however, form infinitives.
b. The word infinitive denotes a verbal form without any limitations (
The infinitive is like a verb herein:
a. It shows the distinctions of voice and tense (but not those of number and person). Having tenses, it can express different stages of action (action simply occurring, continuing, or finished); whereas the corresponding substantive sets forth the abstract idea without these distinctions. Contrast ποιεῖν, ποιήσειν, ποιῆσαι, πεποιηκέναι with ποίησις
b. It can have a subject before it and a predicate after it, and it can have an object in the genitive, dative, or accusative like the corresponding finite verb. Infinitives scarcely ever stand in the subjective genitive; and the object of an infinitive never stands in the objective genitive.
c. It is modified by adverbs, not by adjectives.
e. It forms clauses of result with ὥστε, and temporal clauses with πρίν, etc.1968
The infinitive is like a substantive herein:
a. It may be the subject or object of a verb.
b. With the (neuter) article it shows all the case forms (except the vocative): τὸ (τοῦ, τῷ, τὸ) λύ_ειν, λύ_σειν, etc.
c. It may be governed by prepositions: πρὸ τοῦ λύ_ειν.1969
The infinitive was originally a verbal noun in the dative (in part possibly also in the locative) case. The use to express purpose ( cross2008) is a survival of the primitive meaning, from which all the other widely diverging uses were developed in a manner no longer always clear to us. But the to or for meaning seen in μανθάνειν ἥκομεν
The infinitive is used as subject, as predicate, and to supplement the meaning of words and clauses.1971
The negative of the infinitive is μή; but οὐ, used with a finite mood in direct discourse, is retained when that mood becomes infinitive in indirect discourse. Sometimes, however, μή is used in place of this οὐ ( cross2723 ff.).
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].