There's no one step solution if you need to change a document to past tense. However, Microsoft Word's Find and Replace function provides a suitable alternative to manually replacing each verb. Unfortunately, there is no way for Word to recognize specific parts of speech. Therefore, you'll need to search manually through the document to identify each verb. Make sure to review your changes to the document to verify that it is completely in past tense.
Select "CNTL-F" to bring up the Navigation pane.
Click the arrow on the text box to open the drop-down box and select the "Replace" option.
Click the "More" button, and check the "Find All Word Forms" box.
Enter the verb you want to change in the "Find What" box, and then enter its past tense form in the "Replace With" text box.
Click the "Replace All" button. Select "OK" in the dialog box that warns you about using the Replace All function with the All Word Forms option. Word will go through the document and make the replacements.
Repeat these steps for all verbs in the document.
Go back to the Replace menu from the Navigation pane, and enter "be" (without quotes) in the "Find What" text box, with the All Word Forms option still enabled.
Enter "was" in the "Replace With" text box and click "Replace All" and "OK" in the subsequent pop-up dialog box.
Read through the document to make sure you've changed all of the verbs and that everything is grammatically correct. Make edits as necessary.