The verb understand has only two forms: understand and understood. All due to the fact that he is one of the many irregular verbs. Accordingly, in the first form it is understand, and understood - in the second and third. This should be easy to remember, since all forms of irregular verbs are special (unlike regular ones, where the second and third forms always end in -ed).
Past Tense
Let's consider the forms understand and understood in all four past tenses.
Past Simple:
He understood the meaning of love. - Use in the past simple changed the first form understand to understood in the second form.
Past Continuous:
I was understanding him at that moment. - In the past continuous tense, the ending -ing and the auxiliary verb was were added to the first form understand, which indicates the duration of the action at some point in the past.
Past Perfect:
Carla had already understood the teacher. - Convert understand to understood with PastPerfect occurs with the addition of the auxiliary verb had, which indicates that the action took place long ago in the past. Note that in this case, understood is in the third form, although it looks no different 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 in the past for some time and stopped in the past.
Present Tense
Understand in the present tense is grammatically a little different from the past, only in meaning.
Present Simple:
Lisa understands English. - In the simple present tense, this verb denotes a constant action or simply a fact. The ending -s in the first form indicates belonging to a third person.
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 present, and the action has just ended, as indicated by the auxiliary verb have (or has in 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 tense from the past continuous perfect in thatthe 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 the 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 an 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 be completed by 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 future perfect continuous tense with three auxiliary verbs will have/has been and ending -ing is also better used with another grammatical stem and a verb in a sentence for better understanding.
