A patent validity opinion is undertaken to determine whether a particular patent or group of patents may be asserted against someone introducing a related new machine, product, process or composition of matter. If for any reason (as discussed below) a patent is not valid, it cannot be infringed. Thus, a patent validity opinion may well be part of an overall patent infringement opinion. (Such a patent validity opinion may lead to a broader search for other patents or publications that may be used to invalidate the patent.)
Therefore, the first step is to determine if the term of the patent has run, or whether the patent still appears to be enforced. This will begin with a determination of when the patent issued (for patents prior to June 8, 1995, their life is 17 years from the date of issue) or when the patent application from which the patent resulted was filed (since June 8, 1995, the life of the patent has been 20 years from the date of filing). You can read more from https://www.hngn.com/articles/227862/20200113/what-can-the-experts-at-inventhelp-do-for-you.htm on this subject.
Thus, if the term of a patent has run, it is no longer valid and cannot be infringed. Further, since December 12, 1980 , United States patents have required maintenance fees to keep them in force. If the maintenance fees have not been timely pay, the patent will have expired and cannot be infringed (subject to the possible reinstatement of the patent to valid status, after which he will again become capable of being infringed – although it cannot be infringed during the period of invalidity).
Thus, if the maintenance fees have not been paid and the patent has expired, the opinion will reflect that the patent is not currently in force and cannot be infringed. If, during the patent application process, a reference (either a U.S. patent or published patent application, a foreign patent or foreign published application, or any other material available to the general public) was not considered, the subject patent may not be valid.
If it can be shown that the subject patent was disclosed in any other reference more than a year prior to the filing date of the subject patent, and that such was never considered during the examination process (or possibly may have been misinterpreted during the examination process), the subject patent may be a candidate for re-examination as you can see from https://millennialmagazine.com/2020/01/13/dont-give-up-on-your-invention-idea-turn-to-inventhelp/ article. During such re-examination, the patent may be upheld or may be rendered invalid. If the latter is the case, it cannot be infringed.