Learn Romanian: El lucrează fluierând o melodie.

It comes from the infinitive a fluiera (to whistle). To form the gerund for most verbs ending in -a, we remove the -a and add the suffix -ând. Because this action happens at the same time as the main verb (lucrează), it translates nicely to the English -ing form (whistling).

Would fluierând change if the subject was plural, like 'they work whistling'?

No, it stays exactly the same! The Romanian gerund (ending in -ând or -ind) is invariable. This means it never changes its form to match the subject's gender or number. Whether it is 'I', 'she', or 'we' doing the action, you will always use fluierând.

Why is there an â in the suffix of fluierând instead of an î?

Both â and î make the exact same sound in Romanian. The rule is simply about spelling: you use î at the very beginning or end of a word, and â everywhere in the middle of a word. Since the sound is in the middle of the gerund suffix, we spell it with â.

Why is it o melodie and not un melodie?

Melodie is a feminine noun in Romanian. You can often guess this because it ends in the letters -ie. Because it is feminine, we use the feminine indefinite article o (meaning 'a' or 'an') instead of the masculine and neuter article un.