By 'principal parts of verbs' we mean the most important forms of any verb, especially the one we need to remember.
Out of all the possible forms of any given verb, we have a smaller group called the 'parts of a verb'.
The 'principal parts' refers to a still smaller group of verb forms, which users of English commit to memory.
When we use a verb in a sentence, we use different forms of it for different purposes. Suppose we wanted to use the verb 'to write.' The available forms of that verb would be:
These are called the parts of the verb write.
The forms given below are combinations of the above 'parts of the verb' with helping verbs, whether primary or modal.
am writing, is writing, are writing, was writing, were writing, have written, has written, had written, will write, will be writing, will have written, has been writing, have been writing, had been writing, will have been writing, am written, is written, etc.
See the number of forms! And the list is not complete; there are still more...! Almost all verbs have these many forms.
This sounds like bad news to those who want to learn English grammar.
But, here's the good news...
You don't have to remember all the verb forms!
You need to remember only the principal parts of verbs.
The principal parts of a verb are the minimum number, i.e. three or one, that you need to memorize in case of irregular verbs or regular verbs respectively.
The group of verbs called Regular are those which follow fixed rules for changing into other forms.
So, you need to remember only one form of verbs which belong to this group.
If we remember the verb-form 'play' alone, we can derive all the other forms from it by adding s, ed, or ing, as in plays, playing, played...and we can then get all the multi-word forms too.
If a verb does not follow fixed rules for a change to even one other form, then that verb is called an Irregular Verb.
The verbs sing, bring, and hit are irregular verbs for they do not follow any one rule of change to a particular form (here, the past tense).
The minimum number of forms we have to remember for these irregular verbs are three. For the verb 'to sing', we need to memorize the three forms 'sing, sang, sung' (the principal parts).
The remaining forms can be derived from these one or three forms according to definite patterns, so that we get all the five parts of that verb.
So, when we talk of Principal Parts of Verbs, we mean only the one or three forms, which we need to remember.
How many forms have we to remember for the verb 'be'?
'To be' is an irregular verb, but we need to remember more than the three forms.
The single-word verb forms of 'to be' are eight in number. They are:
am, is, are, was, were, be, being, been. You probably know most of them already.