"I light them."
Translation:Je les allume.
In French, the direct object pronouns me, te, le, la, nous, vous and les go before the conjugated verb.
I recommend that you use the lesson notes on the desktop version of Duolingo as they are very helpful in explaining concepts like this. Here is another link with more information on pronouns: https://www.frenchtoday.com/blog/french-grammar/understanding-french-pronouns