Fill in the blanks with appropriate modals:

Our ancestors ……………………. use kerosene lamps a few years ago.

had to

The modal had to is the most appropriate modal to fill in the blank. The modal had to is used to express obligation in the past. In this case, our ancestors had to use kerosene lamps a few years ago because there was no other source of light available.

The other options are incorrect.

• Must is used to express certainty or strong obligation. In this case, our ancestors did not have to use kerosene lamps, but they did.
• Ought to is used to express advice or a suggestion. In this case, the speaker is not suggesting that our ancestors had to use kerosene lamps.
• Have to is used to express obligation in the present. In this case, our ancestors do not have to use kerosene lamps anymore because there are other sources of light available.