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).
When to use the Imperfect Tense- The imperfect is commonly used to express: 1  Habits or customs in the past. 2  Describe people, animals, things, and with descriptive situations. 3  To tell age, dates, days, names and hours.  What had been happening - Usually expressed in Spanish,HacÃ­a + time + que + imperfect.