Requirement Specification (RS) presents a baseline for the validation/verification step. Therefore, errors within the specification phase involve huge financial burdens (release delay, system cost raise). In fact, the RS step is tedious and an error-prone since the user usually have to handle abstract notation as well as mathematical-based languages within this step. To deal with this issue, we present a pattern-based method for assisting the user during the RS. Indeed, this method predates by defining a new typology taking into account all the common temporal requirements one may meet when dealing with critical systems. Then, in order to provide the user with specification means which are at the same time simple, intuitive and rigorous, we have developed a literal word-based formal grammar able to express all the types of the identified requirements. Furthermore, a generic repository of observation patterns relative to the new time constraint taxonomy is proposed. Concretely, to check the temporal aspects of a given specification, the observation patterns relative to the identified and extracted requirements are instantiated to obtain appropriate observers which will play the role of watch-dog for the system to check.