"I am working."
Translation:मैं काम कर रहा हूँ।
It's always हूँ (which is the form of होना when conjugated with मैं). Just 'हू' is not a word.
Also, though we write हूँ with an 'n' at the end, you don't pronounce the 'n'. It is telling you that the 'ū' vowel sound has to be nasalised. That is what the 'moon-dot' symbol implies.