All verbs have four principal parts: a base form, a present participle, a simple past form, and a past participle. All the verb tenses are formed from these principal parts.
Present perfect tense: Use the present perfect tense to express an action or a condition that occurred at some indefinite time in the past. You form the present perfect tense by using has or have with the past participle of a verb: has permitted, have cut
Present tense: The present tense expresses a constant, repeated, or habitual action or condition. It can also express a general truth or a condition that exists only now. It is sometimes used in historical writing to express past events and, more often, in poetry, fiction, and journalism (especially in sports writing to convey to the reader a sense of being there.
Here is an illustration of the principal parts of verbs: BASE FORM open, fall, cry, speak, be. PRESENT PARTICIPLE opening, falling, crying, speaking, being.
PAST FORM opened, fell, cried, spoke, was, were, PAST PARTICIPLE opened, fallen, cried, spoken, been
You can use the base form (except the base form of be) and the past form by themselves as main verbs. To function as the simple predicate in a sentence, the present participle and the past participle must always be preceded by one or more auxiliary verbs.
EG Doors open. [base or present form] Doors opened. [past form] Doors are opening. [present partoiciple with the auxiliary verb are] Doors have opened. [past participle with the auxiliary verb have]