When is what looks like an infinitive, not an infinitive?

i

When it is a verbal noun or gerund. In this case, participle verbs function like nouns.
Burning (verbal noun) is a form of combustion.

ii

When it is a verbal adjective. Here a participle functions like an adjective.
Burning (verbal adjective) events can be controlled for environmental purposes.

iii

When it is an infinitive which functions like a noun.
To burn (infinitive used as a noun) is essential (the noun here is the equivalent of the verb, as the verb ‘to be’ is a verb of incomplete predication which acts as an equals sign between the two elements of the sentence).

iv

When it is an infinitive functioning like an adjective.
S/He needs a match (noun modified by the verb) to burn (verb acting as an adjective).

v

When it is an infinitive which functions like an adverb.
S/He organised (verb modified by the infinitive) the wherewithal to burn (infinitive used as an adverb).