Past Perfect Tense (Explanations to given exercise)

Suggested Explanations:

• I had not felt like stepping outdoors for several days now.  

The `several days' indicate that whatever had happened took place over a period of time.  To demonstrate the `feeling' that lasted for `several days', the past perfect tense, `had not felt' is used.

• This indiscriminate burning had gone on for too long, I thought to myself at that time.

`Too long' indicates that the `indiscriminate burning' took place over a period of time.

• Indeed, it was a welcomed respite from the hazy conditions that we had been experiencing for the last few days. 

Here there are two verbs, namely `was' and `had been experiencing'.  So, in order to differentiate between the timings of the occurrences of the two events that are connect to each other and had already taken place, or are in the past, the past perfect tense is used to indicate which of the two took place first. In this case, the hazy conditions that `we had been experiencing' took place first before the welcomed respite (`it was'). 

• I wanted to breathe in the air that was fresher than it had been previously.

There are three verbs in the statement above, namely `wanted', `was' and `had been'. To differentiate between the timings of two inter-related occurrences, the perfect tense is used for the first related occurrence.

There is a comparison between the quality of the air at two moments in the past. The air had gradually become fresher. When it was less fresh, the past perfect tense is used to indicate what the quality of the air was before it became fresher later.