They have not caught the thieves.

1. The thieves have not been caught by them.
2. The thieves had not been caught by them.
3. The thieves could have not been caught by them.
4. The thieves would have not been caught by them.

Present perfect passive negative correctly transforms “They have not caught” to “The thieves have not been caught by them.” Structure: object (thieves) becomes subject + have + not + been + past participle (caught). The plural subject “thieves” requires “have” rather than “has.” This form emphasizes the thieves’ ongoing escape status, focusing attention on them rather than the unsuccessful catchers.