A participle can function as a substantive. Usually it is preceded by an article.
The article which defines the participle can either individualise or generalise it. For participles which are indefinite (mainly in the plural) the article is not obligatory.
In poetic language many substantival participles are used as more unusual formulations: ἡ τεκοῦσα 'the mother' (properly 'the bearer', instead of ἡ μήτηρ), οἱ τεκόντες 'the parents' (properly 'the begetters').
Substantival participles are often best translated as nouns or relative clauses, as in ὁ φεύγων 'the fugitive' or οἱ σωθείς 'the man who is saved'.
As stated, substantival participles are usually accompanied by an article: ὁ ἐνθάδε οἰκῶν 'the person who lives here' is more common than ἐνθάδε οἰκῶν 'someone who lives here'.
If the verbal nature of a participle has been significantly weakened, the participle may be accompanied by modifiers.
The negation is οὐ when the participle refers to a specific referent (individualising definite article – τούτων τῶν ἄνδρων ὁ (οὐ) λέγων ‘the one of the men who speaks / does not speak’), and μή when it refers to the entirety of a group (generalising definite article – ὁ (μὴ) βουλόμενος ‘anyone who wishes / does not wish’).
In the first two books of Thucydides 11,6% of the participles are substantival (Coppieters, apud Duhoux).