Final answer:
The answers are 'had never bought' and 'had always rented', corresponding to past perfect tense used for actions that occurred before another action in the past.
Step-by-step explanation:
The correct answers for the sentence completions are: Tom and Barbara had a hard time buying their first house last year because they had never bought a house before then. Until that time, they had always rented an apartment. The correct option that fills the blanks appropriately is a) had bought / had rented.
In English, when discussing past actions that occurred before another action in the past, we use the past perfect tense. The past perfect tense is formed with had plus the past participle of the verb. In this case, since Tom and Barbara had never experienced purchasing a house before a certain point in the past, we say 'had never bought'. Similarly, because they were always renting up until that point, we say 'had always rented'.