Hi. Γειά σας
How is the gerund formed in Greek? And if it doesn't exist, what constructions are used to replace it?
The gerund, as in a verb that acts like a noun, is usually expressed as "the [fact] that I/you/he ...", i.e. you have to choose a person.
You can't just say "Smoking is harmful" but you have to say Το να καπνίζεις βλάπτει την υγεία σου "The fact that you smoke harms your health" or Το να καπνίζει κανείς βλάπτει την υγεία του "The fact that someone smokes harms his health".
So here, it's το να + verb.
Sometimes the το is not needed, e.g. Μου αρέσει [το] να κολυμπάω "I like swimming = I like [the fact] that I swim".
And in case you were asking about the present participle (which, by coincidence, looks like the gerund in English):
As an adverb, it's the unchangeable -οντας/-ώντας, e.g. Ήρθε στο δωμάτο τραγουδώντας "He/She came into the room singing".
Greek doesn't have continuous tenses, so there's no direct equivalent of "I am singing" or "He was playing".