f. The writer (to think) of retiring when he (to get) an idea for a new book.

The sentence is aimed at using the correct verb forms in the context provided. The correct forms of the verbs in parentheses should reflect the timing and tense appropriately.

The appropriate completion of the sentence would be:

"The writer was thinking of retiring when he got an idea for a new book."

Here’s a breakdown of the reasoning:

1. Identify the Tenses: The first verb, "to think," refers to an ongoing action that was happening in the past, hence the past continuous form "was thinking" is used. The second verb, "to get," refers to a completed action also in the past, which takes the simple past form "got."

2. Contextual Understanding: The sentence suggests that the writer was considering retirement when he suddenly received inspiration for a new project. This creates a contrast between his previous thoughts and the new idea that interrupted these thoughts.

Overall, using the past continuous and simple past tenses appropriately captures the sequence and nature of events effectively.