Learn Turkish: Biz odayı mum ile aydınlatmıyorduk, sadece oturuyorduk. - We were not lighting the room with a candle, we were just sitting.

It combines the verb stem aydınlat (to light) with the negative suffix -mı, the continuous suffix -yor, the past tense suffix -du, and the personal ending -k for we. Notice that the negative suffix for this verb would normally be -ma, but the -yor suffix always narrows the vowel before it, turning -ma into -mı.

Why does oda (room) have the -yı ending here?

Because it is the specific direct object of the verb. Oda ends in a vowel, so it takes the buffer letter y, followed by the accusative case ending -ı to make odayı (the room). If it were just oda, it would mean lighting a room in general, not the specific room we are talking about.

Could I combine mum and ile into one word?

Yes, you will often see ile attached directly to the previous word. Because mum ends in a consonant, it would become mumla (with a candle). Both mum ile and mumla are perfectly correct, but the combined form is much more common in spoken Turkish.

Do we need the -k ending on oturuyorduk if the sentence already starts with ?