Principal parts of a regular verb will appear something like this:
pando, pandere, pandi, passus
The first principal part is the 1st person singular, present, active, indicative form-- that is, the form that translates into English as "I verb."
The second principal part is the present active infinitive of the verb. It is from this infinitive that the present stem is obtained.
The third principal part is the 1st person singular, perfect, active indicative form-- that is, the form that translates into English as "I verbed" or "I have verbed."
The fourth principal part is a special verbal noun called the supine. From this perfect passive participles are obtained.
Sometimes a verb may lack a third and/or fourth principal part, meaning that it is either deponent or simply cannot be conjugated in those forms (or is defective). At other times, the third-person singular form will be given instead of the first-person form, meaning the verb is impersonal, or only exists in the third person singular.