For a patent to be infringed it must be current (formally: "in force"). Normally this means that the patent must be less than 20 years old and that the relevant annual tax (renewal fee) must have been paid.
In general terms a patent is infringed if, without the consent of the owner, the invention defined by the patent is: made, sold, offered for sale, used, kept or imported. The detailed rules are, inevitably, complex.
There are some exemptions, for example things which are done privately and for purposes which are not commercial.