Question

Since 1980, several economies in developed nations (are experiencing, have experienced) declines and recoveries.

Short answer

'Since 1980, several economies in developed nations have experienced declines and recoveries.'

Step-by-step solution

Identify and Understand the Context

Look at the time reference - 'Since 1980'. It is important to recognize that the time period mentioned is still ongoing - it began in 1980 and continues until now.

Identify the Correct Tense

In English grammar, if an action started in the past and continues to the present, the present perfect tense, which takes the form 'have/has + past participle', is used to represent this. In this case, the past participle of 'experience' is 'experienced'.

Complete the Sentence

Replace the bracketed section in the original sentence with the correct tense. The finished sentence should read: 'Since 1980, several economies in developed nations have experienced declines and recoveries'.