End face mechanical seal - Seal categories. 1  All mechanical seals must contain the four elements described above but the way those functional elements are arranged may be quite varied. 2  Mechanical seals are generally classified into two main categories: Pusher or Non-Pusher.
The past modal perfect tense begins with any past tense modal (could, should, would, might) followed by âhave,â plus a past tense action verb (regular verbs ending with â-edâ and many irregular verbs ending with-en).
1 Uses of the imperfect subjunctive. 2  The imperfect subjunctive is used in the same type of situations in which the present subjunctive is used, except that the governing verb is typically in a past tense (e.g., the preterit, imperfect, past perfect, conditional, conditional perfect, or one of the past subjunctives): Uses of the imperfect subjunctive. 2  The imperfect subjunctive is used in the same type of situations in which the present subjunctive is used, except that the governing verb is typically in a past tense (e.g., the preterit, imperfect, past perfect, conditional, conditional perfect, or one of the past subjunctives):