The understand verb has only two forms: understand and understood. All due to the fact that he is one of many irregular verbs. Accordingly, in the first form it is understand, and understood - in the second and third. This should simply be remembered, since all forms of irregular verbs are special (unlike regular ones, where the second and third forms always end with -ed).
Past tense
Consider the understand and understood forms in all four past times.
Past Simple:
He understood the meaning of love. - Use in the simple past tense changed the first form of understand to understood in the second form.
Past Continuous:
I was understanding him at that moment. - In the past long time, the ending -ing and the auxiliary verb was were added to the first form of understand, which indicates the duration of the action at some point in the past.
Past Perfect:
Carla had already understood the teacher. - The conversion of understand to understood at Past Perfect occurs with the addition of the auxiliary verb had, which indicates that the action has long been completed in the past. Note, in this case understood is in the third form, although in appearance it does not differ from the second.
Past Perfect Continuous:
Mike had been understanding her reasons until she made that fatal mistake. - The auxiliary verb had been and the ending -ing indicate that the action took place for some time in the past and ceased in the past.
Present tense
Understand in the present tense is slightly different from the past, only in meaning.
Present Simple:
Lisa understands English. - In a simple present tense, this verb denotes a permanent action or simply a fact. The ending -s in the first form indicates belonging to a third party.
Present Continuous:
Jane is not understanding you now. - The auxiliary verb is and the ending -ing indicate that the action is happening right now.
Present Perfect:
I have already understood the book. “Here, even though the verb is in the third form, the time is still real, and the action has just ended, as indicated by the auxiliary verb have (or has with the third person).
Present Perfect Continuous :
He has been understanding the rule since it changed the last time. - The auxiliary verb has been and the ending -ing distinguish this time from the past continuous perfect in that the action is still ongoing.
Future tense
The future tense already has some other differences from the past and present tenses, which are reflected in the change of auxiliary modal verbs.
Future Simple:
Ann promises she will understand him. - And again, the simple first form in a simple future tense, only the modal verb will was added.
Future Continuous :
Maybe Kate will be understanding him next time he'll be explaining it. - The old scheme with the auxiliary verb and the ending -ing to indicate a long time remained, only the modal verb will was added, since this is the future tense.
Future Perfect:
Hope Jake will have understood Christa when she'll end her story. - This time is already more difficult, and for its correct use, the presence of a second verb in the sentence is recommended. Will have and the third form of the verb show that the action will end at some point in the future.
Future Perfect Continuous:
They will have been understanding the meaning of life until they'll disappoint in it. - The most cumbersome time of the future perfect long with three auxiliary verbs will have / has been and ending -ing is also better to use with another grammatical base and a verb in the sentence for better understanding.