Complex sentence: Some single-celled eukaryotes live in places where oxygen is scarce or non-existent; in these circumstances, mitochondria cannot function, but hydrogenosomes are able to "burn" carbohydrates and other fuels to produce ATP without using oxygen.
Split into simpler sentences: Some single-celled eukaryotes live in places where oxygen is scarce or non-existent; these eukaryotic cells use hydrogenosomes instead of mitochondria to produce ATP. Without abundant oxygen mitochondria do not function, but hydrogenosomes are able to "burn" carbohydrates and other fuels to produce ATP without using oxygen.