Choice of tenses (Present Perfect - “has been” VS Present Simple - “is”)

Question

To carry out the task I should fill up the blanks with a correct verb form. Take a look at the given sentences:

"They feel sure they (arrest) the rest of the gang by the end of the month. This (is) the biggest art theft operation to be uncovered since 1974 when police (catch) a gang which (steal) over a million pounds worth of paintings from galleries all over the country."

There have been a lot of similar posts so far. I can't make up my mind which variant would be better to put down, though.

Would you mind to making sure if I have done it in a proper way?

"They feel sure they will have arrested (this task will be already done when there is the end of the month) the rest of the gang by the end of the month. This is (the operation is taking place at this time; different preparation are being made to catch the mobsters) / has been (we're talking about the experience of police since the stated year) the biggest art theft operation to be uncovered since 1974 when police caught (it's clear) a gang which had stolen (by 1974) over a million pounds worth of paintings from galleries all over the country."

I have doubts about ("is"/"has been"), but the whole passage needs to be looked through as well.

Answer 1

Any of the verbs mentioned would be grammatically acceptable in either simple or perfect aspect except "catch," which must be in the simple past. However, the perfect forms seem less idiomatic than the simple forms.

The future perfect is usually used in conjunction with another verb that is in the simple future or implies futurity. "Yes, he will likely finish X before she will have finished Y." But in the example the only other verb explicitly present, "feel," is in the present tense and relates to the present, and the arresting will occur after, not before, the current feeling.

In the case of "is" versus "has been," either is grammatical, but "is" seems somewhat more idiomatic because the intent is to compare the present with 1974, not the intervening time.

In the case of "stole" versus "had stolen," both would be idiomatic, but "stole" would be more common because there is no need to reference the sequence in time. It is obvious that the thieving occurred before capture.

This is a very hard exercise. I suspect other native speakers may disagree with me. To sum up my answer, the past and future perfect tenses tend to be used in English only when sequence is both relevant and otherwise unclear.