Time to send in an amendment
Once an applicant receives an Office action, he or she may need to make amendments to the application.

Amendments are always allowed after the first or second Office action. In addition, they may also be accepted even before the first Office action has ever been mailed, provided certain conditions are met.

Amendments that are sent in before the mailing of the first Office action are termed preliminary amendments. These amendments may be made after or at the time the application is filed. Preliminary amendments filed along with the application are only acceptable when they are used to cancel some of the claims. Otherwise, the amendments must be incorporated into the application.

One reason an applicant might want to cancel their claims is because the filing fee is calculated based upon the number of claims. An applicant may be ready to file their application and realize they can't afford the filing fee due to the number of claims. Therefore, they might opt to cut out a few claims in order to lower the required fees. The PTO will usually accept preliminary amendments up until the first Office action is completed. An examiner doesn't want to re-work a finished first Office action due to the applicant's procrastination.

There are a couple of other times an amendment may be sent in if certain conditions are met. One of these is after the Final Rejection has been mailed. This is only acceptable if the amendment places the application in condition for allowance or in better shape for an appeal. As you might guess, it is even more frowned upon to send in an amendment after the Notice of Allowance has been mailed. In these cases, the applicant will have to explain why the amendment was not sent in earlier, and even then, they may or may not be taken into consideration by the PTO.

Rules
We've alluded to this fact before; amendments may not ever, ever include new matter. Material not disclosed in the original application may not be added in later. There are more than a few examples of just how strict the PTO is on this matter. Deleting a step from a method in the claims will be considered new matter and therefore will not be acceptable. Adding specific percentages or compounds that the disclosure did not originally contain is also not allowed. Instead, you may add new matter into an application by filing a C-I-P application.

The PTO gets picky with what they consider an acceptable format for amendments. For example, if you have to make amendments to both the claims and the specification, each of the two sections should begin on a separate sheet. Furthermore, amendments are never partially entered into an application by the examiner; they are always entered in their entirety, or not at all.

Amendments to the claims are the most complex, but there are specific rules required for amendments to any section in the application.  These help streamline the process and serve as a record for what has been changed in the application.