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).
For example, a sentence that lacks a subject and begins with a present-tense verb is likely imperative, and a sentence that begins with if and uses past-tense verbs is likely subjunctive. Linguists have defined dozens of moods used in languages throughout the world, but English only uses three.