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).
Verbs-Simple Present Tense. Verbs have different forms, called tenses. The tense of a verb tells us when the action happens. In these lessons, we will learn. Related Topics: More English Lessons. 1  when to use the Present Tense. 2  the different spellings for present tense verbs.