Add -οντας to the present stem to make an invariable form meaning "while/by doing": μιλώ → μιλώντας. It never changes shape, and its subject must be the same as the main verb's; passive verbs use the -όμενος participle instead, which does agree like an adjective.
Examples
- Μιλώντας επίσημα, αποφεύγουμε τις αργκό εκφράσεις.
- Speaking formally, we avoid slang expressions.
- Διαβάζοντας το κείμενο, κατάλαβα το ύφος του συγγραφέα.
- Reading the text, I understood the author's tone.
- Χαμογελώντας, μου απάντησε με λεπτή ειρωνεία.
- Smiling, he answered me with subtle irony.
The mistake to avoid
Giving the -οντας clause its own subject — if the doers differ, you need a full clause with καθώς or ενώ.
Where this fits in
Έκφραση και απόχρωση, Greek C1: Master register, idiomatic expression, and emotional nuance.