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).
In the English language, we have different verb tenses to demonstrate time, and we have verb moods that indicate a state of being or reality. These moods are: indicative, imperative, interrogative, conditional and subjunctive.